cxgb4: Avoids race and deadlock while freeing tx descriptor

There could be race between t4_eth_xmit() and t4_free_sge_resources() while
freeing tx descriptors, take txq lock in t4_free_sge_resources(). We need
to stop the xmit frame path which runs in bottom half context while
unloading the driver using _bh variant of the lock. This is to prevent
the deadlock between xmit and driver unload.

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