Also implement the behaviour reported in Linux's ftdi_sio.c whereby if an invalid
data_bits value is provided then the hardware defaults to using 8.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20201027150456.24606-8-mark.cave-ayland@ilande.co.uk
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>