media: stb6100: mark a currently unused function as such

The stb6100_normalise_regs() function is not used with current
boards, but the driver says that some devices could need it.
Ok, we could simply drop it, but as there's a macro to mark
unused functions, use it.

This should shut up a clang warning.

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