media: cec-core.rst: fix warnings with Sphinx 3.0+

The new C domain code on Sphinx 3 is a lot more pedantic.
It only accepts real functions declared as c:function.

So, declarations like this are not valid:

	.. c:function::
	        int (*adap_enable)(struct cec_adapter *adap, bool enable);

Also, no blank lines are allowed after ".. c:function:", and
continuation lines should be like:

	.. c:function:
		int (void foo, \
		     int bar);

Change the logic there, in order to avoid lots of warnings
when built with Sphinx 3.x.

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