iwlwifi: fix iwl_mac_add_interface handler

This patch fixes iwl_mac_add_interface.

1. Currently only one interface is supported, instead of silently retuning
 0 now it returns  -EOPNOTSUPP (By Johannes Berg)
2. It enables changing mac address from user space (By Ian Schram)

Signed-off-by: Tomas Winkler <tomas@localhost.localdomain>
Cc: Zhu Yi <yi.zhu@intel.com>
Cc: Reinette Chatre <reinette.chatre@intel.com>
Cc: linux-wireless <linux-wireless@vger.kernel.org
Cc: Ian Schram <ischram@telenet.be>
Cc: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>