[MAC80211]: improve locking of sta_info related structures

The sta_info code has some awkward locking which prevents some driver
callbacks from being allowed to sleep. This patch makes the locking more
focused so code that calls driver callbacks are allowed to sleep. It also
converts sta_lock to a rwlock.

Signed-off-by: Michael Wu <flamingice@sourmilk.net>
Signed-off-by: Jiri Benc <jbenc@suse.cz>
Signed-off-by: John W. Linville <linville@tuxdriver.com>