[media] dvb-usb/Kconfig: auto-select XC4000 tuner for dib0700

Automatically select the xc4000 module for dib0700 if the tuner selection is
not customized.

Signed-off-by: Istvan Varga <istvan_v@mailbox.hu>
Cc: Patrick Boettcher <pboettcher@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>