iwlwifi: pcie: add RTPM support when wifi is enabled

Enable runtime power management (RTPM) for PCIe devices and implement
the corresponding functions to enable D0i3 mode when the device is
idle.

Additionally, remove some unnecessary #ifdef's because the RTPM code
will not be called if runtime PM is not configured.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>