rt2800usb: Add support for 2001:3c1e (D-Link DWA-125 rev B1) USB Wi-Fi adapter

D-Link DWA-125/B1 is a relatively new USB Wi-Fi adapter, using a
Ralink chipset supported by the rt2800usb driver. Currently, to work
around the problem (it's missing in all present kernel versions,
up to and including 3.7.x), I had to add this to /etc/rc.local:

echo 2001 3c1e >> /sys/bus/usb/drivers/rt2800usb/new_id

After that, the device works without problems. Been using it for over
a week with no bugs in sight.

The attached patch is trivial and simply adds the new USB ID to the
list of devices handled by rt2800usb.

Signed-off-by: Maia Kozheva <sikon@ubuntu.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>