b43legacy: Clean up beacon IRQ

This patch ports commit c97a4ccc "b43: Fix beacon BH update" to
b43legacy. It fixes beacon updating in the bottomhalf. In case the device
is busy, we will defer to later in the IRQ handler.

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>