media: adv7842: get rid of two unused functions

Those functions just call a V4L2 core macro, and aren't used
anywhere. Get rid of them, in order to avoid clang warnings with
W=1.

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