iwlwifi: pcie: add workaround for power gating in integrated 22000

Add a workaround that forces power gating to be enabled on integrated
22000 devices.  This improves power saving in certain situations.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>