V4L/DVB (12403): em28xx: properly reports some em2710 chips

As reported by hermann pitton <hermann-pitton@arcor.de>, some devices
has a different chip id for em2710 (likely the older ones):

em28xx: New device @ 480 Mbps (eb1a:2710, interface 0, class 0)
em28xx #0: Identified as EM2710/EM2750/EM2751 webcam grabber (card=22)
em28xx #0: em28xx chip ID = 17

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>