cxgbe(4): Use hw_port and tx_chan correctly all over the driver

The port's tx channel and its hardware index are different things that
happen to have the same value on NICs using T4 through T6. This is
going to change so use the correct spelling all over the driver.
Specifically, the firmware expects the hw port index in all of these:

• PCIe channel for queues
• TX interface for TX CPLs
• PORTID in FW_PORT commands
• PORTID in t4_alloc_vi
• flowid in all the firmware flowc WRs
• mbox commands that deal with the tx scheduler

Also, create a port_id lookup table for hw_port, just like the one for
tx_chan. Use it to lookup the port softc in the port_info handler.

MFC after: 3 days
Sponsored by: Chelsio Communications