media: usb: dvb-usb-v2: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix multiple
warnings by explicitly adding a couple of break statements instead of
just letting the code fall through to the next case.

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



Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>