ALSA: hda - hdmi: Fix missing ELD change event on plug/unplug

The ELD ALSA control change event is sent by hdmi_present_sense() when
eld_changed is true.

Currently, it is only true when the ELD buffer contents have been
modified. However, the user-visible ELD controls also change to a
zero-length value and back when eld_valid is unset/set, and no event is
currently sent in such cases (such as when unplugging or replugging a
sink).

Fix the code to always set eld_changed if eld_valid value is changed,
and therefore to always send the change event when the user-visible
value changes.

Signed-off-by: Anssi Hannula <anssi.hannula@iki.fi>
Cc: David Henningsson <david.henningsson@canonical.com>
Cc: <stable@vger.kernel.org> # 3.9+
Signed-off-by: Takashi Iwai <tiwai@suse.de>