b43: Accessing the TSF via mac80211

This allows the mac80211 high level code to access the TSF. This is e.g. needed for BSSID merges in the IBSS mode.

The second version adds locking and removes the now unnecessary debugfs entries.

Thanks to Michael Buesch! :)

Signed-off-by: Alina Friedrichsen <x-alina@gmx.net>
Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>