northbridge/via/vx800: Fix out-of-bounds read due to off-by-one

Change-Id: Ia7fda59b60b2148dd4d246686bd94d2334b23eb5
Found-by: Coverity Scan
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/6485
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>