b43legacy: include full 64-bit timestamp in monitor mode

When monitor mode is enabled, this will make b43legacy read out
the full 64-bit MAC time from the chip for each received packet.

This patch has been ported from b43.

Signed-off-by: Stefano Brivio <stefano.brivio@polimi.it>
Cc: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>