cxgbe(4): Get the number of usable traffic classes from the firmware.

Recent firmwares are able to utilize the traffic classes of tx channels
that were previously unused. This effectively doubles the number of
traffic classes available per port for 2 port cards. Stop using the raw
per-channel value in the driver and ask the firmware for the number of
usable traffic classes instead.

MFC after: 2 weeks
Sponsored by: Chelsio Communications