ALSA: vx222: fix null-ptr-deref

a recent refactor created a null pointer vx in snd_vx222_probe().
The vx pointer should have been populated in snd_vx222_create() as
suggested in earlier version, otherwise vx->core.ibl.size will throw an
error.

[    1.298398] BUG: kernel NULL pointer dereference, address: 00000000000001d8
[    1.316799] RIP: 0010:snd_vx222_probe+0x155/0x290 [snd_vx222]

Fixes: 3bde3359

 ("ALSA: vx222: Allocate resources with device-managed APIs")
Signed-off-by: Tong Zhang <ztong0001@gmail.com>
Link: https://lore.kernel.org/r/20210907014746.1445278-1-ztong0001@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>