media: i2c: ov7251: Add V4L2_CID_VBLANK

This is a raw sensor so should be implementing V4L2_CID_VBLANK
instead of the frame_interval ops, as per docs at
https://www.kernel.org/doc/html/latest/driver-api/media/camera-sensor.html#frame-interval-configuration



This driver already implemented the frame_interval ops, so
removing them would be a regression.
Implement V4L2_CID_VBLANK, with the frame_interval ops setting
that control.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>