wl12xx: Revert "wl12xx: disable auto-arp"

This reverts commit e5e2f24b.

The encryption consideration on auto-arp configuration,
along with a fw fix, seem to resolve the crashes that
occured when auto-arp was enabled, so we can re-enable it now.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>