media: i2c: imx290: Correct range for V4L2_CID_GAIN to 0-238

The datasheet lists the gain as being 0.0 to 72.0dB in 0.3dB steps, which
makes 238 steps total.
Correct the 0-72 range defined in the driver.

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