USB: serial: ftdi_sio: ignore baud_base changes

The TIOCSSERIAL error handling is inconsistent at best, but drivers tend
to ignore requests to change parameters which cannot be changed rather
than return an error.

The FTDI driver ignores change requests for all immutable parameters but
baud_base so return success also in this case for consistency.

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