iwlwifi: mvm: add const to thermal_cooling_device_ops structure

Declare thermal_cooling_device_ops structure as const as it is only passed
as an argument to the function thermal_cooling_device_register and this
argument is of type const. So, declare the structure as const.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>