b43legacy: fix TBTT and PU timings

This fixes some timings for pre-TBTT and synthetic PU.
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>