ath9k: fix timeout on stopping rx dma

It seems that using ath9k_hw_stoppcurecv to stop rx dma is not enough.
When it's time to stop DMA, the PCU is still busy, so the rx enable
bit never clears.
Using ath9k_hw_abortpcurecv helps with getting rx stopped much faster,
with this change, I cannot reproduce the rx stop related WARN_ON anymore.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Cc: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>