V4L/DVB (8950): xc5000: prevent an OOPS if analog driver is unloaded while digital is in use

Prevent an OOPS if xc5000_attach was called by tuner.ko before being called by
the DVB adapter driver. The OOPS occurs when a digital tune request is made
after tuner.ko is unloaded.

When tuner.ko is unloaded, it takes the xc5000_config structure with it.

Rather than storing a pointer to the xc5000_config structure, just store the
if_khz and tuner_callback inside the xc5000_priv internal state structure.

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