V4L/DVB (10920): v4l2-ioctl: fix partial-copy code.

The code to optimize the usercopy only checked the ioctl NR field. However,
this code is also called for non-V4L2 ioctls (either private or ioctls from
linux/dvb/audio.h and linux/dvb/video.h for decoder drivers like ivtv).

If such an ioctl has the same NR as a V4L2 ioctl, then disaster strikes.

Modified the code to check on the full command ID.

Thanks to Martin Dauskardt for tracing the ivtv breakage to this particular
change.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>