ALSA: hwdep: Take private_data as drvdata for sysfs

For referring to a different object from sysfs ops, take hwdep
private_data as stored via dev_set_drvdata() at creating the device
object.  In that way, the same sysfs ops can be used by different
device types.

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