Since interrupts are already masked by the hardware, there's no need to
discard interrupt bits in the ISR itself.  Also, in ath5k_beacon_config
we mask off a couple of bits without locking, so doing this mask in
software can lead to unhandled beacon timer and beacon miss interrupts.

Changes-licensed-under: 3-Clause-BSD
Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>