fsi: scom: Fix NULL dereference

The chardev conversion forgot to copy the fsi_dev,
silly mistake, compounded by a testing mistake on
my side, this specific driver wasn't being tested
properly.

Fixes: d8f45876 "fsi: scom: Convert to use the new chardev"
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>