ath5k: Use new function to stop beacon queue

ath5k: Use new function to stop beacon queue

 * Since we only use ath5k_hw_stop_tx_dma to stop the beacon
 queue, introduce a new function ath5k_hw_stop_beacon_queue so
 that we can use that instead and have better control. In the future
 we can add more beacon queue specific stuff there (maybe tweak
 beacon timers or something), for now just call ath5k_hw_stop_tx_dma.

 * Also since we don't call ath5k_hw_stop_rx/tx_dma from outside
 dma.c, make them static.

 Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>