iio: core: Mark expected switch fall-through

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

Addresses-Coverity-ID: 1397962
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/industrialio-core.c

@@ -588,6 +588,7 @@ static ssize_t __iio_format_value(char *buf, size_t len, unsigned int type,
 		return snprintf(buf, len, "%d", vals[0]);
 	case IIO_VAL_INT_PLUS_MICRO_DB:
 		scale_db = true;
+		/* fall through */
 	case IIO_VAL_INT_PLUS_MICRO:
 		if (vals[1] < 0)
 			return snprintf(buf, len, "-%d.%06u%s", abs(vals[0]),