ath9k: Call spin_lock_bh() on btcoex_lock

As generic hw timer interrupt handler is moved to tasklet,
we no more need to call spin_lock_irqsave().

Signed-off-by: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>