b43: Fixup set_key handling

This fixes the key handling for mac80211's new key->flags.
It also adds TX locking to the set_key handler and adds a comment why this is required.
This doesn't fix any known bugs.

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