Only mask out MAC interrupt if necessary and re-enable on ifup.  There
could be multiple netdevs going through the same MAC.  MAC interrupts
should not be masked off until the last netdev is downed.
Signed-off-by: NChien Tung <ctung@neteffect.com>
Signed-off-by: NGlenn Streiff <gstreiff@neteffect.com>
Signed-off-by: NRoland Dreier <rolandd@cisco.com>