ath5k: Use new dma_stop function on base.c

 * Since we stop rx/tx dma and pcu durring reset there is no need to
 call ath5k_hw_stop_rx/tx_dma before, also there is no need to call
 them durring stop_locked since we can use ath5k_hw_dma_stop for
 both.

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