For embedded platforms, beacon transmission can be starved when
flooded with data packets.  Prioritize beacons by giving the beacon
queue the first shot when the isr completes.

Changes-licensed-under: 3-Clause-BSD
Signed-off-by: Bob Copeland <me@bobcopeland.com>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>