ath5k: disable irq handling in ath5k_hw_detach()

Once ah has been freed, it may not be accessed.  Set ATH_STAT_INVALID
bit to make the interrupt handler return IRQ_NONE without accessing ah.

This fixes oops on unload with CONFIG_DEBUG_SHIRQ enabled.

Signed-off-by: Pavel Roskin <proski@gnu.org>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>