ASoC: SOF: Drop superfluous snd_pcm_sgbuf_ops_page

ASoC: SOF: Drop superfluous snd_pcm_sgbuf_ops_page

snd_pcm_sgbuf_ops_page is no longer needed to be set explicitly to PCM
page ops since the recent change in the PCM core (*).  Leaving it NULL
should work as long as the preallocation has been done properly.

This patch drops the redundant lines.

(*) 7e8edae39fd1: ALSA: pcm: Handle special page mapping in the
                  default mmap handler

Acked-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20191108094641.20086-6-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>