sb600spi.c: Consolidate smbus dev revision derivation

V.2: Rename 'find_smbus_dev()' -> 'find_smbus_dev_rev()'.

Change-Id: I766b29cc1c7d01aa0bcf6cb9ff5ab73fa1995dcd
Signed-off-by: Edward O'Callaghan <quasisec@chromium.org>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/36420


Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: Nico Huber <nico.h@gmx.de>