mac80211: add op to configure default key id

There are hardwares which support offload of data packets
for example when auto ARP is enabled the hw will send
the ARP response. In such cases if WEP encryption is
configured the hw must know the default WEP key in order
to encrypt the packets correctly.

When hw_accel is enabled and encryption type is set to WEP,
the driver should get the default key index from mac80211.

Signed-off-by: Yoni Divinsky <yoni.divinsky@ti.com>
[cleanups, fixes, documentation]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>