USB: add siemens_mpi usb-serial "stub" driver

This driver got rescued from a few years ago and was requested to be
added.  So I cleaned it up, ported it to the latest kernel version and
here it is.

Cc: Thomas Hergenhahn <thomas.hergenhahn@suse.de>
Cc: Emmanuele <iemmav@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>