iwlwifi: pcie: make iwl_pcie_apm_stop_master() return void

Nothing ever checks the return value of iwl_pcie_apm_stop_master(),
so there's no point in it having one - make it return void.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>