net/mlx4_core: Make sure there are no pending async events when freeing CQ

When freeing a CQ, we need to make sure there are no
asynchronous events (on the ASYNC EQ) that could
relate to this CQ before freeing it.

This is done by introducing synchronize_irq.

Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Ido Shamay <idos@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>