V4L/DVB (9922): em28xx: don't assume every eb1a:2820 reference design is a Prolink PlayTV USB2

Don't operate under the assumption that every device that uses the em2820
default USB ID is a Prolink PlayTV USB.  We have an eeprom hash, so use that,
since otherwise we cannot support other devices with the 2820 default USB ID
(such as the ADS Tech Instant TV USB USBAV-704)

Signed-off-by: Devin Heitmueller <dheitmueller@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>