b43: include full 64-bit timestamp in monitor mode

When monitor mode is enabled, this will make b43 read out the
full 64-bit MAC time from the chip for each received packet.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>