MFC: r343934

• Remove the redundant device disabled hint handling; ever since r241119 that's performed globally by device_attach(9).
• As for the EM-class of devices, em(4) supports multiple queues and MSI-X respectively only with 82574 devices. However, since the conversion to iflib(4), em(4) relies on the interrupt type fallback mechanism, i. e. MSI-X -> MSI -> INTx, of iflib(4) to figure out the interrupt type to use for the EM-class (as well as the IGB-class) of MACs. Moreover, despite the datasheet for 82583V not mentioning any support of MSI-X, there actually are 82583V devices out there that report a varying number of MSI-X messages as supported. The interrupt type fallback of iflib(4) is causing two failure modes depending on the actual number of MSI-X messages supported for such instances of 82583V:
  1. With only one MSI-X message supported, none is left for the RX/TX queues as that one message gets assigned to the admin interrupt. Worse, later on - which will be addressed with a separate fix - iflib(4) interprets that one messages as MSI or INTx to be set up, but fails to actually do so as it has previously called pci_alloc_msix(9). [1, 2]
  2. With more message supported, their distribution is okay but then em_if_msix_intr_assign() doesn't work for 82583V, with the interface being left in a non-working state, too. [3] Thus, let em_if_attach_pre() indicate to iflib(4) to try MSI-X with 82574 only, and at most MSI for the remainder of EM-class devices. While at it, remove "try_second_bar" as it's polarity inverted and not actually needed.
• Remove code from em_if_timer() that effectively is a NOP since the conversion to iflib(4) ("trigger" is no longer read). While at it, let the comment for em_if_timer() reflect reality after said conversion.
• Implement an ifdi_watchdog_reset method which only updates the em(4) "watchdog_events" counter but doesn't perform any reset, so that the em(4) "watchdog_timeouts" SYSCTL (iflib(4) doesn't provide a counterpart) reflects reality and these timeouts add to IFCOUNTER_OERRORS again after the iflib(4) conversion.
• Remove the "mbuf_defrag_fail" and "tx_dma_fail" SYSCTLS; since the iflib(4) conversion, associated counters are disconnected, but iflib(4) provides "mbuf_defrag_failed" and "tx_map_failed" respectively as equivalents.
• Move the description preceding lem_smartspeed() to the correct spot before em_reset() and bring back appropriate comments for {igb,em}_initialize_rss_mapping() and lem_smartspeed() lost in the iflib(4) conversion.
• Adapt some other function descriptions and INIT_DEBUGOUT() use to match reality after the iflib(4) conversion.
• Put the debugging message of em_enable_vectors_82574() (missed in r343578) under bootverbose, too.

PR: 219428 [1], 235246 [2], 235147 [3]
Reviewed by: erj (previous version)
Differential Revision: https://reviews.freebsd.org/D19108