USB: serial: ftdi_sio: fix FTX sub-integer prescaler

The most-significant bit of the sub-integer-prescaler index is set in
the high byte of the baudrate request wIndex also for FTX devices.

This fixes rates like 1152000 which got mapped to 1.2 MBd.

Reported-by: Vladimir <svv75@mail.ru>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=210351


Cc: stable@vger.kernel.org
Signed-off-by: Johan Hovold <johan@kernel.org>