media: vivid: fix assignment of dev->fbuf_out_flags

Currently the chroma_flags and alpha_flags are being zero'd with a bit-wise
mask and the following statement should be bit-wise or'ing in the new flag
bits but instead is making a direct assignment.  Fix this by using the |=
operator rather than an assignment.

Addresses-Coverity: ("Unused value")

Fixes: ef834f78 ("[media] vivid: add the video capture and output parts")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>