e1000: improve and correct link interrupt handling

[marius]

Summary

• In em_intr(), just call em_handle_link() instead of duplicating it.
• In em_msix_link(), properly handle IGB-class devices after the iflib(4) conversion again by only setting EM_MSIX_LINK for the EM-class 82574 and by re-arming link interrupts unconditionally, i. e. not only in case of spurious interrupts. This fixes the interface link state change detection for the IGB-class. [1]
• In em_if_update_admin_status(), only re-arm the link state change interrupt for 82574 and also only if such a device uses MSI-X, i. e. takes advantage of autoclearing. In case of INTx and MSI as well as for LEM- and IGB-class devices, re-arming isn't appropriate here and setting EM_MSIX_LINK isn't either. While at it, consistently take advantage of the hw variable.

PR: 236724 [1]

This revision is intended to obsolete D21769.

Test Plan

This change has been tested to correctly indicate the interface link status for LEM-, EM- and IGB-class devices, both with INTx/MSI and MSI-X for the latter two.