ALSA: control led - improve the set_led_id() parser

It may be possible that the string pointer does not move
when parsing. Add a code which detects this state and
simply break the parser loop in this case.

Signed-off-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20210331180725.663623-1-perex@perex.cz
Signed-off-by: Takashi Iwai <tiwai@suse.de>