USB: serial: fix minor-number allocation

Due to a missing upper bound, invalid minor numbers could be assigned to
ports. Such devices would later fail to register, but let's catch this
early as intended and avoid having devices with only a subset of their
ports registered (potentially the empty set).

Signed-off-by: Johan Hovold <johan@kernel.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>