rt2x00: Allow beacon update without scheduling a work

Since all rt2x00 PCI drivers use threaded interrupts now we don't need
to schedule a work just to update the beacon. The only place where the
beacon still gets updated in atomic context is from the set_tim
callback. Hence, move the work scheduling there.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>