cxgb4: tune burst buffer size for TC-MQPRIO offload

For each traffic class, firmware handles up to 4 * MTU amount of data
per burst cycle. Under heavy load, this small buffer size is a
bottleneck when buffering large TSO packets in <= 1500 MTU case.
Increase the burst buffer size to 8 * MTU when supported.

Also, keep the driver's traffic class configuration API similar to
the firmware API counterpart.

Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>