Ath5k: lock beacons

Beacons setup and config was racy with beacon send. Ensure that
ISR and reset functions see consistent state of bbuf.

Use also dev_kfree_skb_any in ath5k_txbuf_free since we call it
from atomic now.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Cc: Nick Kossifidis <mickflemm@gmail.com>
Cc: Luis R. Rodriguez <mcgrof@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>