ASoC: Intel: mrfld: fix incorrect check on p->sink

The check on p->sink looks bogus, I believe it should be p->source
since the following code blocks are related to p->source. Fix
this by replacing p->sink with p->source.

Fixes: 24c8d141

 ("ASoC: Intel: mrfld: add DSP core controls")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Addresses-Coverity: ("Copy-paste error")
Link: https://lore.kernel.org/r/20191119113640.166940-1-colin.king@canonical.com
Signed-off-by: Mark Brown <broonie@kernel.org>