iwlwifi: pcie: only use d0i3 in suspend/resume if system_pm is set to d0i3

We only need to handle d0i3 entry and exit during suspend resume if
system_pm is set to IWL_PLAT_PM_MODE_D0I3, otherwise d0i3 entry
failures will cause suspend to fail.

This fixes https://bugzilla.kernel.org/show_bug.cgi?id=194791



Signed-off-by: Luca Coelho <luciano.coelho@intel.com>