iwlwifi: make iwl_fill_probe_req static

This function is only used in iwl-scan.c, so
if we move it up a little in the file it can
be made static.

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>