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>

Documentation/video4linux/CARDLIST.em28xx

   0 -> Unknown EM2800 video grabber             (em2800)        [eb1a:2800]
-  1 -> Unknown EM2750/28xx video grabber        (em2820/em2840) [eb1a:2820,eb1a:2860,eb1a:2861,eb1a:2870,eb1a:2881,eb1a:2883]
+  1 -> Unknown EM2750/28xx video grabber        (em2820/em2840) [eb1a:2820,eb1a:2821,eb1a:2860,eb1a:2861,eb1a:2870,eb1a:2881,eb1a:2883]
   2 -> Terratec Cinergy 250 USB                 (em2820/em2840) [0ccd:0036]
   3 -> Pinnacle PCTV USB 2                      (em2820/em2840) [2304:0208]
   4 -> Hauppauge WinTV USB 2                    (em2820/em2840) [2040:4200,2040:4201]
@@ -12,7 +12,7 @@
  11 -> Terratec Hybrid XS                       (em2880)        [0ccd:0042]
  12 -> Kworld PVR TV 2800 RF                    (em2820/em2840)
  13 -> Terratec Prodigy XS                      (em2880)        [0ccd:0047]
- 14 -> Pixelview Prolink PlayTV USB 2.0         (em2820/em2840) [eb1a:2821]
+ 14 -> Pixelview Prolink PlayTV USB 2.0         (em2820/em2840)
  15 -> V-Gear PocketTV                          (em2800)
  16 -> Hauppauge WinTV HVR 950                  (em2883)        [2040:6513,2040:6517,2040:651b]
  17 -> Pinnacle PCTV HD Pro Stick               (em2880)        [2304:0227]

drivers/media/video/em28xx/em28xx-cards.c

@@ -1246,7 +1246,7 @@ struct usb_device_id em28xx_id_table [] = {
 	{ USB_DEVICE(0xeb1a, 0x2820),
 			.driver_info = EM2820_BOARD_UNKNOWN },
 	{ USB_DEVICE(0xeb1a, 0x2821),
-			.driver_info = EM2820_BOARD_PROLINK_PLAYTV_USB2 },
+			.driver_info = EM2820_BOARD_UNKNOWN },
 	{ USB_DEVICE(0xeb1a, 0x2860),
 			.driver_info = EM2820_BOARD_UNKNOWN },
 	{ USB_DEVICE(0xeb1a, 0x2861),