Fetch usb-modeswitch and usb-modeswitch-data from backports.

This adds support for a few devices such as Pantech UMW190 CDMA modem.

config/chroot_apt/preferences

@@ -166,6 +166,14 @@ Package: libservice-wrapper-jni
 Pin: release o=Debian,a=experimental
 Pin-Priority: 999
 
+Package: usb-modeswitch
+Pin: origin backports.debian.org
+Pin-Priority: 999
+
+Package: usb-modeswitch-data
+Pin: origin backports.debian.org
+Pin-Priority: 999
+
 Explanation: weirdness in chroot_apt install-binary
 Package: *
 Pin: release o=chroot_local-packages