codecs/rt5640: Fix restoring of playback volumes

codecs/rt5640: Fix restoring of playback volumes

The playback volumes shall be initialized only during of the first
enable sequence, not during the initialization of speaker/headphones
devices. This fixes sound level restoring by pulseaudio when headphones
are inserted/ejected. Previously sound level was always reset to the
default level on insert/ejection, which is the incorrect behaviour.
The correct behaviour is to restore volume the previous level, i.e.
if sound level was 3% before headphones were ejected, then level should
be restored to 3% when headphones are inserted back.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>