[media] em28xx: add image quality bridge controls

Add the image quality bridge controls contrast, brightness, saturation,
blue balance, red balance and sharpness.
These controls are enabled only if no subdevice provides them.
Tested with the following devices:
"Terratec Cinergy 200 USB"
"Hauppauge HVR-900"
"SilverCrest 1.3MPix webcam"
"Hauppauge WinTV USB2"
"Speedlink VAD Laplace webcam"

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>