rsi: handle station disconnection in AP mode

When sta_remove of mac80211 is called, driver iterates through
list of existing stations to get the station id. Then peer notify
is prepared and send to firmare.

Signed-off-by: Prameela Rani Garnepudi <prameela.j04cs@gmail.com>
Signed-off-by: Amitkumar Karwar <amit.karwar@redpinesignals.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>