V4L/DVB (6852): xc2028: s-code offset should not modify internal control structure

Don't modify the control structure that was provided at attach when applying
an offset to the S-Code, otherwise it will be incorrect on subsequent tunes.

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