cxgb4: when disabling dcb set txq dcb priority to 0

When we are disabling DCB, store "0" in txq->dcb_prio
since that's used for future TX Work Request "OVLAN_IDX"
values. Setting non zero priority upon disabling DCB
would halt the traffic.

Reported-by: AMG Zollner Robert <robert@cloudmedia.eu>
CC: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: Casey Leedom <leedom@chelsio.com>
Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>