ath9k_hw: prevent a fast channel change after a rx DMA stuck issue

If the receive path gets stuck, a full hardware reset is necessary to
recover from it. If this happens during a scan, the whole scan might fail,
as each channel change bypasses the full reset sequence.
Fix this by resetting the fast channel change flag if stopping the
receive path fails.

This will reduce the number of error messages that look like this:
ath: DMA failed to stop in 10 ms AR_CR=0x00000024 AR_DIAG_SW=0x40000020

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