USB: serial: ftdi_sio: clean up ioctl handler

Clean up the ioctl handler and make sure to pass an unsigned-int rather
than serial_struct pointer to the TIOCSERGETLSR helper as this it what
the user argument really is.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>