iwlwifi: move spectrum measurement code to iwl-spectrum.c file

This patch moves spectrum measurement code into iwl-sepctrum.c file.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>