media: v4l2-ioctl.c: OR flags in v4l_fill_fmtdesc(), not don't overwrite

If a driver sets a FMT flag in the enum_fmt op, then that will be
ignored since v4l_fill_fmtdesc() overwrites it again.

v4l_fill_fmtdesc() should OR its flag, not overwrite it.

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