ath9k_hw: fix stopping rx DMA during resets

During PHY errors, the MAC can sometimes fail to enter an idle state on older
hardware (before AR9380) after an rx stop has been requested.

This typically shows up in the kernel log with messages like these:

ath: Could not stop RX, we could be confusing the DMA engine when we start RX up
------------[ cut here ]------------
WARNING: at drivers/net/wireless/ath/ath9k/recv.c:504 ath_stoprecv+0xcc/0xf0 [ath9k]()
Call Trace:
[<8023f0e8>] dump_stack+0x8/0x34
[<80075050>] warn_slowpath_common+0x78/0xa4
[<80075094>] warn_slowpath_null+0x18/0x24
[<80d66d60>] ath_stoprecv+0xcc/0xf0 [ath9k]
[<80d642cc>] ath_set_channel+0xbc/0x270 [ath9k]
[<80d65254>] ath_radio_disable+0x4a4/0x7fc [ath9k]

When this happens, the state that the MAC enters is easy to identify and
does not result in bogus DMA traffic, however to ensure a working state
after a channel change, the hardware should still be reset.

This patch adds detection for this specific MAC state, after which the above
warnings completely disappear in my tests.

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