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>

Documentation/driver-api/media/cec-core.rst

@@ -98,7 +98,7 @@ Implementing the Low-Level CEC Adapter
 The following low-level adapter operations have to be implemented in
 your driver:
 
-.. c:type:: struct cec_adap_ops
+.. c:struct:: cec_adap_ops
 
 .. code-block:: none