ath9k: Use lockless variant to initialize RX fifo

Since the rx_fifo queue is accessed only using the various
lockless SKB queue routines, there is no need to initialize
the lock and __skb_queue_head_init() can be used.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>