iwlwifi: pcie: Control access to the NIC's PM registers via iwl_cfg

Allow a cleaner way to access those hw-dependent registers,
instead of using the product family type etc.

Signed-off-by: Avri Altman <avri.altman@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>