[media] em28xx: add probing procedure for OmniVision sensors

OmniVision sensors are used as well in Empiatech based cameras such as the
"SpeedLink Vicious And Devine Laplace" webcam (EM2765 + Omnivision OV2640).
With this patch applied, OminiVision sensors with 8 bit address and register
width are detected (recent models have a 16 bit address width and use different
client addresses).
The most commonly used sensors (including the ones listed by Empiatech) are
detected properly, although there is no support for them yet.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>