net: usb: dm9601: Add USB ID of Keenetic Plus DSL

[ Upstream commit a609d0259183a841621f252e067f40f8cc25d6f6 ]

Keenetic Plus DSL is a xDSL modem that uses dm9620 as its USB interface.

Signed-off-by: Kamil Lorenc <kamil@re-ws.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>