media: cec: add core error injection support

Add two new ops (error_inj_show and error_inj_parse_line) to support
error injection functionality for CEC adapters. If both are present,
then the core will add a new error-inj debugfs file that can be used
to see the current error injection commands and to set error injection
commands.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>