wl1271: remove unnecessary joins and join only when the bssid changes

We were using the join command to change some settings when the stack asked
us to do it.  In many cases they were not needed (and could cause potential
problems), so they were removed.  In other cases there are ACX commands that
can be used instead of using join to reconfigure.

Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Reviewed-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>