soc-pcm.c :: soc_get_playback_capture() will indicate error
if both playback_only / capture_only were true.

Thus, graph_util_parse_link_direction() which setup playback_only /
capture_only don't need to check it.
And, its return value is not used on existing driver. Let's remove it.
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://msgid.link/r/87a5kah6gm.wl-kuninori.morimoto.gx@renesas.comSigned-off-by: Mark Brown <broonie@kernel.org>