ath5k: use spin_lock_irqsave for beacon lock

ath5k_reset can be called from process context, which in turn can
call ath5k_beacon_config which takes the sc->block spinlock.  Since
it can also be taken in hard irq context, use spin_lock_irqsave
everywhere.  This fixes a potential deadlock in adhoc mode.

Changes-licensed-under: 3-Clause-BSD

Cc: stable@kernel.org
Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>