wl1271: workaround to send a disconnect before rejoining

We don't get any indication from the stack when we have disassociated.  In
wl1271, it is important to send a CMD_DISCONNECT before joining again,
because the firmware cleans some things up.  So we check if we're already
joined and disconnect if that's the case.

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>