V4L/DVB (4656): Fixed dvb_attach for dib3000mc in dibusb

When converting the dib3000mc-driver to dvb_attach I forgot to invert the check for NULL.

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