RDMA/nes: Fix MAC interrupt erroneously masked on ifdown

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: Chien Tung <ctung@neteffect.com>
Signed-off-by: Glenn Streiff <gstreiff@neteffect.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>