ath9k: fix beacon resource related race condition

The beacon tasklet is accesssing the bslot info for beacon generation.
Meanwhile the same slot can be freed on interface deletion.
Current the remove_interface disables the beacon alert after freeing the slot.
This may leads to null pointer access.

This patch disables SWBA and kills the beacon tasklet to prevent access
to the slot to be freed. After releasing the slot, swba will be enabled again
upon the availablity of beaconing interfaces.

Signed-off-by: Rajkumar Manoharan <rmanoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>