ALSA: control: Allow NULL passed to snd_ctl_remove()

There are lots of code checking NULL for kcontrol passed to
snd_ctl_remove() in the caller side.  Let's make snd_ctl_remove()
accepting the NULL kcontrol instead a la free(), so that we can clean
up the caller side.

Link: https://lore.kernel.org/20240617100529.6667-2-tiwai@suse.de


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