media: v4l2-ctrls.c: initialize flags field of p_fwht_params

The V4L2_CID_STATELESS_FWHT_PARAMS compound control was missing a
proper initialization of the flags field, so after loading the vicodec
module for the first time, running v4l2-compliance for the stateless
decoder would fail on this control because the initial control value
was considered invalid by the vicodec driver.

Initializing the flags field to sane values fixes this.

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