rndis_host: support WM6 devices as modems

This patch allows Windows Mobile 6 devices to be used for
tethering -- that is, used as modems.  It was requested by
AdamW in kernel bugzilla:

  http://bugzilla.kernel.org/show_bug.cgi?id=11119

and Mandriva kernel-discuss list.  It is tested and confirmed
to work by Peterl:

  http://forum.eeeuser.com/viewtopic.php?pid=323543#p323543

This patch is based on the patch in the above kernel bugzilla,
which is from the usb-rndis-lite tree.

[ dbrownell@users.sourceforge.net: misc fixes ]
Signed-off-by: Thomas Backlund <tmb@mandriva.org>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>