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.

Sponsored by: Chelsio Communications

(cherry picked from commit 6beb67c7e0ad4c3f8277ed1122ef5efcde0a269c)