ath9k: remove old rx dma stop check optimization

This commit was added to improve reset time on old SoC devices that run
into chip hangs more frequently. However with the more recent addition
of full WMAC reset on these chips, it could be problematic.
Drop this patch to ensure that DMA activity is really stopped before the
chip reset is issued

Signed-off-by: Felix Fietkau <nbd@nbd.name>