media: cec: return -ENODEV instead of -ENXIO if unregistered

If the CEC device is unregistered, then an attempt to open
the device node should result in an -ENODEV error instead of
-ENXIO.

Document this as well in cec-func-open.rst.

This is consistent with the error code returned by other
file operations such as ioctl.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>