snd_cx18_pcm_ioctl() does nothing but calling the default handler.
Now PCM core accepts NULL as the default ioctl ops, so let's drop
altogether.

Cc: Andy Walls <awalls@md.metrocast.net>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>