cxgb4: fix possible deadlock

t4_wr_mbox_meat_timeout() can be called from both softirq
context and process context, hence protect the mbox with
spin_lock_bh() instead of simple spin_lock()

Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>