mac80211: call drv_get_tsf() in sleepable context

The call to drv_get/set_tsf() was put on the workqueue to perform tsf
adjustments since that function might sleep. However it ended up inside
a spinlock, whose critical section must be atomic. Do tsf adjustment
outside the spinlock instead, and get rid of a warning.

Signed-off-by: Thomas Pedersen <thomas@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>