b43legacy: fix upload of beacon packets to the hardware

This fixes uploading of the beacon data and writing of the TIM and DTIM offsets.

The patch by Michael Buesch has been ported to b43legacy.

Signed-off-by: Stefano Brivio <stefano.brivio@polimi.it>
Signed-off-by: John W. Linville <linville@tuxdriver.com>