media: dvb-usb: vp702x: use an enum for the device number

The device number is currently a value that needs to be the same
on two separate tables, but the code doesn't actually enforce it,
leading to errors as boards get added or removed.

Fix it by using an enum.

Link: https://lore.kernel.org/linux-media/13bf6aab3909fae5da4c9a24c114b15e76abd146.1648499509.git.mchehab@kernel.org


Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>