USB: Ignore storage device in modem mode on DWN-652

D-Link DWN-652 in Modem mode exposes 3 interfaces
- First one is the USB storage one
- Second one is for both control and connection
- Third one is unknown

This patch avoids usb-storage trying to switch again when already in
modem mode, and exposes only 2 ttyUSB instead of 3 by not attaching
to the storage interface

Signed-off-by: Pascal Terjan <pterjan@mandriva.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>