cxgb4: Mask out interrupts that are not enabled.

There are rare cases where a PL_INT_CAUSE bit may end up getting
set when the corresponding PL_INT_ENABLE bit isn't set.

Signed-off-by: Vishal Kulkarni <vishal@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>