media: cec-core.rst: don't use c:type for structs

The new C domain code on Sphinx 3 doesn't allow anymore
to use c:type:: for structs.

Now that cdomain.py has backward support, let's use
c:struct:: instead.

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