mwl8k: handle station database update for AP's sta entry via ->sta_notify()

Inserting and removing a hardware station database entry for the AP
when we are in managed mode is currently done in ->bss_info_changed().

To prepare for adding AP mode support, implement the ->sta_notify()
driver method, and let that handle inserting and removing the hardware
station database entry for our AP instead.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>