ath5k: Stop PCU on reset

 * Stop PCU receive logic (DRU) durring reset
 We need to be sure pcu is not active when trying to stop rx dma
 right now this is done on ath5k_reset (base.c) but later we are
 going to clean it up.

Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>

Signed-off-by: John W. Linville <linville@tuxdriver.com>