ALSA: hda - hdmi: Do not expose eld data when eld is invalid

Previously, it was possible to read the eld data of the previous
monitor connected. This should not be allowed.

Also refactor the function slightly.

Signed-off-by: David Henningsson <david.henningsson@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>