usb-serial: show port number in sysfs

Some usb-serial devices (e.g. certain Edgeport models) have more than
one serial port on the same USB device/interface.

Currently the only way to distinguish these ports in userspace is by
their minor device number: the driver makes them consecutive and in
stable order.

However, for the purpose of stable naming with udev this is
insufficient: when udev handles the ADD event for one of the ports it
doesn't know what minor number the other one has.

To make stable naming easier, export the port number via sysfs.

Signed-off-by: Roman Kagan <rkagan@sw.ru>
Signed-off-by: Dmitry Guryanov <dimak@dgap.mipt.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>