V4L/DVB (13105): dvb-usb: add fe_ioctl_override callback to dvb_usb_adapter_properties

Allow dvb-usb adapter drivers to specify a fe_ioctl_override callback
to enable device-specific handling of DVB frontend operations.

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