rt2x00: Enable missing interrupts in rt61pci

We're handling both, the CSR_BEACON_DONE and the CSR_TWAKEUP interrupts
in rt61pci. However, these interrupts are masked out by default. Fix
this.

Found via pure code inspection.

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>