[media] v4l2-dev.c: add debug sysfs entry

Since this could theoretically change the debug value while in the middle
of v4l2-ioctl.c, we make a copy of vfd->debug to ensure consistent debug
behavior.

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