iwlwifi: fix locking in iwl_mac_add_interface

The corresponding iwl_mac_remove_interface only
acquires the mutex, leading me to believe that
the spinlock is not necessary. However, this
doesn't actually acquire the mutex around the
vif pointer check and assignment, fix that.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>