iio: health: max30102: Mark expected switch fall-throughs

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

Notice that in this particular case, I placed the "fall through"
annotation at the bottom of the case, which is what GCC is expecting
to find.

Addresses-Coverity-ID: 1458342 ("Missing break in switch")
Addresses-Coverity-ID: 1458345 ("Missing break in switch")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>