USB: ftdi_sio: remove obsolete port data refcounting

Remove the port data refcounting and release the private data
explicitly at port remove.

The port data refcounting was used to make sure the port data was not
freed until the last tty reference was closed. Since moving over to tty
ports, the underlying assumptions are no longer valid as close is now
called as part of tty port shutdown, which can occur before the final
tty reference is dropped on device disconnect.

This means that the private port data refcounting is now completely
useless, as the last reference will always be dropped on port_remove.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>