ALSA: hwdep: Embed struct device

Like the previous patch, this one embeds the device object into hwdep
object.  For a proper object lifecycle, it's freed in the release
callback.

This also allows us to create sysfs entries via passing to the groups
field of the device without explicit function calls.  Since each
driver can see the device and touch its groups field directly, we
don't need to delegate in hwdep core any longer.  So, remove the
groups field from snd_hwdep, and let the user (in this case only
hda_hwdep.c) modify the device groups.

Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Signed-off-by: Takashi Iwai <tiwai@suse.de>