iwlwifi: default WEP HW encryption

This patch adds HW encryption support in default WEP mode.
When no key mapping key/pairwise key is used. The key is broadcast key
is used as default/global/static key.
This code assumes that group cast key is added after pairwise key.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>