ASoC: omap-mcpdm: Mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Addresses-Coverity-ID: 1369526 ("Missing break in switch")
Addresses-Coverity-ID: 1369529 ("Missing break in switch")
Addresses-Coverity-ID: 1451415 ("Missing break in switch")
Addresses-Coverity-ID: 115103 ("Missing break in switch")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Mark Brown <broonie@kernel.org>