mac80211: use separate spinlock for sta flags

David Ellingsworth posted a bug that was only noticable on UP/NO-PREEMPT
and Michael correctly analysed it to be a spin_lock_bh() section within
a spin_lock_irqsave() section. This adds a separate spinlock for the
sta_info flags to fix that issue and avoid having to take much care
about where the sta flag manipulation functions are called.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reported-By: David Ellingsworth <david@identd.dyndns.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>