media: cec: cec-adap.c: improve CEC_MSG_FL_REPLY_VENDOR_ID check

The new CEC_MSG_FL_REPLY_VENDOR_ID flag only makes sense in combination
with CEC_MSG_VENDOR_COMMAND_WITH_ID. So rather than reporting an error
if that flag is set with another command, just clear the flag instead.

Only keep the message length check, since otherwise the flag would not
make sense.

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