wl1271: use join command with dummy BSSID

When we need to change the channel before association, we have to send a join
command with a valid BSSID.  With this patch we use 0baddeadbeef as the
BSSID.  There are ongoing discussions with TI to get this done in a cleaner
way.

When we go back to idle, we issue a CMD_DISCONNECT to make sure the firmware
stops listening to the channel and cleans things up internally.

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>