fmap: Fix length calculation in error message

Change-Id: Ie0f448970de6a7829f304448e0835eaeb7d103a3
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/c/30152
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: David Hendricks <david.hendricks@gmail.com>
1 file changed