rsi: handle station connection in AP mode

Station structures are maintained in driver with required
fields. When mac80211 callback sta_add is called, driver
iterates through list of connected stations to check available
index and assigns station id which is important for further
communication to that station. Then peer notify frame is send
to firmware to inform the firmware about new station connection.

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>