rt2x00: Mask out unused interrupts in rt2800pci

We don't use all available interrupts in rt2800pci. Mask out all unused
interrupts to avoid waking up without having anything to do.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>