cxgb4: Reset dcb state machine and tx queue prio only if dcb is enabled

When cxgb4 is enabled with CONFIG_CHELSIO_T4_DCB set, VI enable command
gets called with DCB enabled. But when we have a back to back setup with
DCB enabled on one side and non-DCB on the Peer side. Firmware doesn't
send any DCB_L2_CFG, and DCB priority is never set for Tx queue.
But driver resets the queue priority and state machine whenever there
is a link down, this patch fixes it by adding a check to reset only if
cxgb4_dcb_enabled() returns true.

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>