V4L/DVB (10181): v4l2-device: Fix some sparse warnings

/home/v4l/master/v4l/v4l2-device.c:32:2: warning: Using plain integer as NULL pointer
/home/v4l/master/v4l/v4l2-device.c:64:2: warning: Using plain integer as NULL pointer

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>