[media] ov2640: fix vflip control

Enabling vflip currently causes wrong colors.
It seems that (at least with the current sensor setup) REG04_VFLIP_IMG only
changes the vertical readout direction.
Because pixels are arranged RGRG... in odd lines and GBGB... in even lines,
either a one line shift or even/odd line swap is required, too, but
apparently this doesn't happen.

I finally figured out that this can be done manually by setting
REG04_VREF_EN.
Looking at hflip, it turns out that bit REG04_HREF_EN is set there
permanetly, but according to my tests has no effect on the pixel readout
order.
So my conclusion is that the current documentation of sensor register 0x04
is wrong (has changed after preliminary datasheet version 2.2).

I'm pretty sure that automatic vertical line shift/switch can be enabled,
too, but until anyone finds ot how this works, we have to stick with manual
switching.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>