ALSA: pcm: Allow NULL ioctl ops

Currently PCM ioctl ops is a mandatory field but almost all drivers
simply pass snd_pcm_lib_ioctl.  For simplicity, allow to set NULL in
the field and call snd_pcm_lib_ioctl() as default.

Link: https://lore.kernel.org/r/20191117085308.23915-4-tiwai@suse.de


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