drivers: video: ov7670: Set default format to RGB565 QVGA

The default format for ov7670 is currently VGA YUYV and it counts on the
smartdma to reset the format to RGB565 QVGA when get_format() is called.

Recently, set_format() is decoupled from get_format() so this assumption
is nolonger correct.

Set the default format to RGB565 QVGA instead.

Signed-off-by: Phi Bang Nguyen <phibang.nguyen@nxp.com>