km/boards: fix typo for MAC address offset config option

This patch only fixes a typo.

Signed-off-by: Holger Brunck <holger.brunck@ch.abb.com>
CC: Stefan Roese <sr@denx.de>
CC: Valentin Longchamp <valentin.longchamp@ch.abb.com>
Reviewed-by: Stefan Roese <sr@denx.de>