ASoC: wm8978: enable symmetric rates

wm8978 needs .symmetric_rates = 1.
The playback/capture will be strange without this patch
when it used asymmetric rate in same time

Tested-by: Yusuke Goda <yusuke.goda.sx@renesas.com>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@linaro.org>