ASoC: ti: davinci-mcasp: Fix fall-through warning for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a fallthrough; statement.

Link: https://github.com/KSPP/linux/issues/115


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Acked-by: Peter Ujfalusi <peter.ujflausi@gmail.com>
Link: https://lore.kernel.org/r/20210604072714.GA244640@embeddedor


Signed-off-by: Mark Brown <broonie@kernel.org>