V4L/DVB (6470): Avoid breaking compilation

The next patchset series will change i2c structs inside tuner. This patch
avoids breaking bissect, by commenting the still unused tuner xc2028.

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