ALSA: control - add the missing prev_lops2 initialization

ALSA: control - add the missing prev_lops2 initialization

As static analysis reported, the prev_lops2 should contain
the previous lops2 pointer in snd_ctl_disconnect_layer().

Link: https://lore.kernel.org/alsa-devel/96e9bd5c-c8db-0db8-b393-fbf4a047dc80@canonical.com/
Fixes: c639d7443710 ("ALSA: control - add layer registration routines")
Signed-off-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20210331180702.663489-1-perex@perex.cz
Signed-off-by: Takashi Iwai <tiwai@suse.de>