[media] cxusb: Use enum to represent table offsets rather than hard-coding numbers

Use enum to represent table offsets rather than hard-coding numbers to avoid
problems with the numbers becoming out of sync with the table.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>