[media] em28xx: don't treat device as webcam if an unknown sensor is detected

With an unknown sensor, norm_maxw() and norm_maxh() return 0 as max.
height and width values, which causes a devide by zero in size_to_scale().
Of course we could use speculative default values for unknown sensors,
but the chance that the device works at this resolution without any
driver/setup is very low and therefore not worth the efforts.
Instead, just don't treat the device as camera.
A message will then be printed to the log that the device isn't supported.

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