b43legacy: add definitions for MAC control register

This adds some definitions for the MAC control register and uses them. This is
basically no functional change.

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>