b43legacy: Fixes for beaconing

This patch ports the beaconing fixes from commit a82d992261f "b43: Beaconing
fixes" to b43legacy. Basically it prevents the card from triggering the beacon
IRQ over and over again.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Acked-by: Michael Buesch <mb@bu3sch.de>
Tested-by: David Ellingsworth <david@identd.dyndns.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>