ath9k: clean up hardware code for beacon handling

ath9k: clean up hardware code for beacon handling

The registers TBTT_TIMER ,DMA_BEACON_ALERT ,NEXT_SWBA are need to be
configured only for AP and IBSS mode.

SWBA register is used for generating software interrupts so that beacon
frames will be created by the software.DMA beacon alert register is
to indicate the hardware to DMA the contents of beacon buffer to PCU buffer
and TBTT to start transmitting the packet buffer to the base band.
Clearly these things are not needed for station/monitor mode so
remove configuring them.

Cc: doug dahlby <ddahlby@atheros.com>
Signed-off-by: Mohammed Shafi Shajakhan <mshajakhan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>