media: v4l2-ioctl: create helper to fill in v4l2_standard for ENUMSTD

Prepare for adding a new IOCTL VIDIOC_SUBDEV_ENUMSTD which would
enumerate the standards for a subdevice by breaking out the code which
could be shared between the video and subdevice versions of this IOCTL.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
[hans.verkuil@cisco.com: fixed 'sdandard' typos in v4l2-ioctl.h]
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>