rt2x00: Reorganize beacon handling

With the new beacon handling from mac80211 we can
reorganize the beacon handling in rt2x00 as well.
This patch will move the function to the TX handlers,
and move all duplicate code into rt2x00queue.c.

After this change the descriptor helper functions
from rt2x00queue.c no longer need to be exported
outside of rt2x00lib and can be declared static.

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