wlcore: set default_wep_key when configured

When associating to an AP with WEP set the
default key upon association by implementing
the set_deafult_key_idx op.

Fixes auto-arp sent with wrong key_idx bug.

Signed-off-by: Yoni Divinsky <yoni.divinsky@ti.com>
Signed-off-by: Eyal Shapira <eyal@wizery.com>
Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>