[media] xc4000: Fix get_frequency()

The programmed frequency on xc4000 is not the middle
frequency, but the initial frequency on the bandwidth range.
However, the DVB API works with the middle frequency.

This works fine on set_frontend, as the device calculates
the needed offset. However, at get_frequency(), the returned
value is the initial frequency. That's generally not a big
problem on most drivers, however, starting with changeset
6fe1099c

, the frequency drift is taken into account at
dib7000p driver.

This broke support for PCTV 340e, with uses dib7000p demod and
xc4000 tuner.

Cc: stable@vger.kernel.org
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>