iio: adc: cpcap: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a break statement instead of letting the code fall
through to the next case.

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://github.com/KSPP/linux/issues/115
Link: https://lore.kernel.org/r/b3c1c3f9c76f2f0e832f956587f227e44af57d3d.1605896060.git.gustavoars@kernel.org


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>