ath9k: Use lockless variants for the RX fifo queue

The RX fifo can be accessed from the common tasklet or it can
be reaped/cleaned when RX is stopped, which is done when doing
a reset or channel change - this happens in process context.

Since it is ensured that there are no pending tasklets when
stopping RX and cleaning the FIFO, there is no need to use
SKB queue functions which take internal locks.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Acked-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>