[media] media: i2c: adv748x: Map v4l2_std_id to the internal reg value

The video standard was not mapped to the corresponding value of the
internal video standard in adv748x_afe_querystd, causing the wrong
video standard to be selected.

Fixes: 3e89586a ("media: i2c: adv748x: add adv748x driver")
[Kieran: Obtain the std from the afe->curr_norm]

Signed-off-by: Simon Yuan <simon.yuan@navico.com>
Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>