ALSA: control - off by one in store_mode()

If count is 16 then this will put the NUL terminator one element beyond
the end of the array.

Fixes: cb17fe00 ("ALSA: control - add sysfs support to the LED trigger module")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/YGcDOtrimR46vr0k@mwanda


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