ath5k: use tasklet_hi_schedule for beacon queue

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>