iwlwifi: separate thermal throttling function

"Thermal Throttling" is an advance feature which only available for
newer _agn devices. Move from iwl-core to iwl-agn for better code
organization.

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