[media] em28xx: improve em2710/em2820 distinction

Chip id 18 is used by the em2710 and em2820.
The current code assumes that if the device is a camera, the chip is an em2710
and an em2820 otherwise.
But it turned out that the em2820 is also used in camera devices.
"Silvercrest 1.3 MPix" webcams for example are available with both chips.
Fortunately both variants are using different generic USD IDs which give us a
hint about the used chip.

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