wl1271: use acx_rx_config instead of join when updating filters

We shouldn't use a join command to change the filter settings while
associated.  The right way to do it is to use ACX_RX_CFG.

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>