V4L/DVB (3202): Initialize the dvb-usb-device-pointer to NULL

- Initialize the the dvb-usb-device-pointer in the -init function to NULL,
to be sure that a dvb-usb-device was really initialized.

Signed-off-by: Patrick Boettcher <pb@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@brturbo.com.br>