USB: serial: ftdi_sio: clean up TIOCSSERIAL

The TIOCSSERIAL implementation needs to compare the old flag and divisor
settings with the new to detect ASYNC_SPD changes, but there's no need
to copy all driver state to the stack for that.

While at it, unbreak the function parameter list.

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