We have currently sysfs attributes for each hwdep, but basically these
should belong to the codec itself, per se.  Let's add them to the
codec object while keeping them for hwdep as is for compatibility.

While we are at it, split the sysfs-related stuff into a separate
source file, hda_sysfs.c, and keep only the stuff necessary for hwdep
in hda_hwdep.c.
Signed-off-by: Takashi Iwai <tiwai@suse.de>