media: uapi/linux/cec.h: cec_msg_set_reply_to: zero flags

The cec_msg_set_reply_to() helper function never zeroed the
struct cec_msg flags field, this can cause unexpected behavior
if flags was uninitialized to begin with.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Fixes: 0dbacebe ("[media] cec: move the CEC framework out of staging and to media")
Cc: <stable@vger.kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>