ALSA: hda/generic - Don't override power_filter when power_save_node is set

Currently the generic parser sets codec->power_filter when
power_save_node flag is set.  But this overrides the existing filter
that has been already set by the codec driver, thus it looses some
features.  Instead, set the default power_filter only when it's not
set yet.

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