ALSA: hda/generic - Make snd_hda_gen_path_power_filter() always applicable

Add the check of power_save_node flag at the beginning of the function
so that it skips the rest if the flag isn't set.  In this way, we can
call this function safely no matter whether the widget power-saving is
really used or not.

Signed-off-by: Takashi Iwai <tiwai@suse.de>