ALSA: control_led - fix the stack usage (control element ops)

ALSA: control_led - fix the stack usage (control element ops)

It's a bad idea to allocate big structures on the stack.
Mark the variables as static and add a note for the locking.

Fixes: 406e467c087d ("ALSA: control - add generic LED trigger module as the new control layer")
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20210414105858.1937710-1-perex@perex.cz
Signed-off-by: Takashi Iwai <tiwai@suse.de>