ALSA: led: Use restricted type for iface assignment

Fix a sparse warning that complains about the inconsistent type
assignment for iface, which is a restricted type of
snd_ctl_elem_iface_t.

Fixes: a135dfb5 ("ALSA: led control - add sysfs kcontrol LED marking layer")
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/202111201028.xduVYgH5-lkp@intel.com
Link: https://lore.kernel.org/r/20211123170247.2962-1-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>