wl12xx: don't regulate links when a single STA is connected

When operating as AP track the number of connected stations. When a
single STA is connected don't regulate the PS status of the link.
Since this is the only STA connected, there's no point holding space in
FW for other links. This will speed up communications with a single
connected STA in PSM.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>