[media] v4l2-ioctl: log buffer type 0 correctly

If userspace passed the invalid buffer type 0 to the kernel, then the
kernel log would show 'type=(null)' since there was no string defined
for type 0. Fix this.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>