V4L/DVB (12138): em28xx: add support for Silvercrest Webcam

V4L/DVB (12138): em28xx: add support for Silvercrest Webcam

This webcam uses a em2710 chipset, that identifies itself as em2820,
plus a mt9v011 sensor, and a DY-301P lens.

It needs a few different initializations than a normal em28xx device.

Thanks to Hans de Goede <hdegoede@redhat.com> and Douglas Landgraf
<dougsland@redhat.com> for providing the acces for the webcam during
this weekend, I could make a patch for it while returning back from
FISL/Fudcom LATAM 2009.

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