media: dvb-core: dvb_frontend: address some clang warnings

The typecasts at the dvb-core generate clang warnings when W=1
is enabled.

The warns are harmless, but they cause the build to break with
CONFIG_WERROR, so do the cast on a way that it won't produce
warnings anymore.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>