iwlwifi: pcie: only apply retention workaround on 9000-series A-step

Due to a hardware issue, certain power saving had to be
disabled. However, this issue was fixed in B-step, so the
workaround only needs to apply to A-step.

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