uas: Remove cmnd reference from the cmd urb

It is not strictly necessary for the cmd urb to have a reference to the
cmnd, and without this reference it becomes easier to drop all references to
a cmnd on an abort.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>