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: NYoni Divinsky <yoni.divinsky@ti.com>
Signed-off-by: NEyal Shapira <eyal@wizery.com>
Signed-off-by: NEliad Peller <eliad@wizery.com>
Signed-off-by: NLuciano Coelho <coelho@ti.com>