RDMA/nes: Stop spurious MAC interrupts

Mask off MAC interrupts on netdev_stop to prevent spurious MAC interrupts
on unload/reload of iw_nes.

Signed-off-by: Sweta Bhatt <sweta.bhatt@einfochips.com>
Signed-off-by: Chien Tung <ctung@neteffect.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>