Like other drivers, expose the ELD bytes via a control element so that
user-space can parse it.

For the simplicity, the code to register the ctl elements is
refactored using an array.  Also, since ELD ctl read copies the bytes
also during disconnection, clear the ELD bytes at hot-unplug, in order
to avoid the leak of the previous bogus ELD.
Signed-off-by: Takashi Iwai <tiwai@suse.de>