media: i2c: adv748x: fix HDMI field heights

The ADV748x handles interlaced media using V4L2_FIELD_ALTERNATE field
types.  The correct specification for the height on the mbus is the
image height, in this instance, the field height.

The AFE component already correctly adjusts the height on the mbus, but
the HDMI component got left behind.

Adjust the mbus height to correctly describe the image height of the
fields when processing interlaced video for HDMI pipelines.

Fixes: 3e89586a ("media: i2c: adv748x: add adv748x driver")

Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
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@s-opensource.com>