media: i2c: imx258: Make HFLIP and VFLIP controls writable

The sensor supports H & V flips, but the controls were READ_ONLY.

Note that the Bayer order changes with these flips, therefore
they set the V4L2_CTRL_FLAG_MODIFY_LAYOUT property.

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