ASoC: soc-core: Set dpcm_playback / dpcm_capture

When converting a normal link to a DPCM link we need
to set dpcm_playback / dpcm_capture otherwise playback/capture
streams will not be created resulting in errors like this:

[   36.039111]  sai1-wm8960-hifi: ASoC: no backend playback stream

Fixes: a655de80

 ("ASoC: core: Allow topology to override machine driver FE DAI link config")
Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>
Link: https://lore.kernel.org/r/20191204151333.26625-1-daniel.baluta@nxp.com
Signed-off-by: Mark Brown <broonie@kernel.org>