mwl8k: convert to new station add/remove callbacks

This converts mwl8k to use the new station
add/remove callbacks instead of using the
old sta_notify callback.

The new callbacks can sleep, so a lot of
code can be removed now.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>