V4L/DVB (6875): tuner-xc2028: ATSC requires 1.75 MHz tuning offset

In my testing yesterday, I was using a scan file tailored specifically
for a unique test situation -- As it turns out, this scan file was bad,
and I will use the one included inside dvb-apps for testing for now on.

I've tested with other ATSC tuners just to confirm, using:

us-ATSC-center-frequencies-8VSB

Anyhow, as it turns out, the tuner-xc2028 *does* require a tuning offset
for ATSC.  Even though the linux-dvb api passes in center frequencies
from userspace, apparantly the xceive firmware is already factoring in
the tuning offset to center.

In order to make the device function using the same scan files /
channels.conf configurations as other atsc devices, we must offset by
1.75 MHz.

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>