wl12xx: implement sta_state callback

Implement sta_state callback instead of the
sta_add/remove callbacks.

Update the fw regarding peer state and ht caps
only after the station was authorized. Otherwise,
the fw might try establishing BA session before
the sta is authorized.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>