USB: ftdi_sio: fix TIOCSSERIAL baud_base handling

Return EINVAL if new baud_base does not match the current one.

The baud_base is device specific and can not be changed. This restores
the old (pre-2005) behaviour which was changed due to a
misunderstanding regarding this fact (see
https://lkml.org/lkml/2005/1/20/84).

Reported-by: Torbjörn Lofterud <torbjorn@pi.nxs.se>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>