ALSA: hda - fix ELD memory leak

memset(eld) clears eld->proc_entry which will leak the struct
snd_info_entry when unloading module.

Fix it by
- memset only the fields before eld->eld_buffer
- set eld->eld_valid to true _after_ all eld fields have been filled

Cc: <stable@kernel.org>
Cc: Pierre-louis Bossart <pierre-louis.bossart@intel.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>