media: adv7604: mark unused functions as such

There are several unused helper macros there, meant to parse some
fields. As those actually help to document the hardware, the
better is to keep them.

However, it generates clang warnings with W=1, causing build to
break with CONFIG_WERROR.

So, add __always_unused to fix such warnings.

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