ASoC: intel: skylake: Remove superfluous bus ops

The bus_core_ops values set in skl driver are same as the default
values, so we can drop it and pass NULL to snd_hdac_ext_bus_init()
instead.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20191212191747.19995-1-tiwai@suse.de
Signed-off-by: Mark Brown <broonie@kernel.org>