wl1271: Use the ARP configuration function from mac80211

This patch updates the driver to use the ARP configuration function from the
mac80211. Also, clean up IPv6 support from the ACX function as ARP is not
used with that protocol version.

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