iwlagn: move scan code to scan file

Since the driver split there's no longer a need
to have the scan code scattered across multiple
files, so move it all back to iwl-scan.c

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