[media] em28xx: move the probing of Micron sensors to a separate function

Other sensors like the ones from OmniVision need a different probing procedure,
so it makes sense have separate functions for each manufacturer/sensor type.

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