ALSA: control_led - fix initialization in the mode show callback

The str variable should be always initialized before use even if
the switch covers all cases. This is a minimalistic fix: Assign NULL,
the sprintf() may print '(null)' if something is corrupted.

Signed-off-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20210614071710.1786866-1-perex@perex.cz


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