USB: ftdi_sio: use port_probe / port_remove thereby fixing access to the latency_timer

Convert all the port specific code in attach / shutdown to use the new
port_probe / port_register callbacks from device_register /
device_unregister allowing adding the sysfs attributes to be added at
the correct time and to the serial port device itself, instead of to
the unadorned usb device, avoiding a NULL dereference.

Signed-off-by: Jim Radford <radford@blackbean.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>