- added calc_num_ports() callback so that driver can override the
	  fixed num_ports value after querying the device.
	- split startup() callback into probe() and attach() in anticipation
	  of the driverfs api changes
	- probe() is called before the usb_serial structure is set up,
	  and can be used to download firmware to a device, and other
	  early initialization.
	- attach() is called after the usb_serial structure is completely
	  setup, allowing the device to create private structures, and
	  have full access to the device.