V4L/DVB (6645): xc2028: allow selection of D2633 firmware

Add a bit to select D2633 DTV firmware to struct xc2028_ctrl, so that it can
be enabled via .set_config.

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