USB: usbtmc: add device quirk for Rigol DS6104

Recently we purchased the Rigol DS6104 and when I try to operate it from
my Linux pc, everything works well with the default usbtmc driver,
except when I want to download a big datachunk like a screenshot. This
bitmapfile has a size of 1152054 bytes but I receive a smaller file and
no new packets can be read.

When I took a look at the driver source, I found this "Rigol quirk" and
I added the id of the new DS series oscilloscopes to this list. I
compiled it and loaded the new driver and now everything seems to work
fine.

Signed-off-by: Teunis van Beelen <teuniz@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>