media: cec-ioc-g-mode.rst: be more specific when EPERM is returned

Document which capability is required, rather than just saying
that "root permissions" are required.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>