media: bcm2835-unicam: Re-fetch mbus code from subdev on a g_fmt call

The sensor subdevice may change the Bayer order if a H/V flip is
requested after a s_fmt call.  Unicam g_fmt must call the subdev get_fmt
in case this has happened and return out the correct format 4cc.

Signed-off-by: Naushir Patuck <naush@raspberrypi.com>