USB: serial: keyspan_pda: drop redundant usb-serial pointer

Drop the redundant struct usb_serial pointer from the driver port data.

Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>