V4L/DVB (6644): xc2028: use correct offset into scode firmware

When validating and loading SCODE firmware we need to take into account the
two-byte size header before each entry.

Signed-off-by: Chris Pascoe <c.pascoe@itee.uq.edu.au>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>