dtc2278: fix ->io_32bit handling

On DTC2278 32-bit I/O has to be enabled for both devices on the port
so always enable it during init time and disallow further changes.

Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>