iwlwifi: move iwl_sta_id_or_broadcast to user

There's only one user, so the function
can be moved into the correct file. It
also loses an argument along the way.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>