Don't try to compute I/O for empty sub buses.

Don't try to compute I/O for empty sub buses.

I am not sure if the sub bus being 0 is a problem, or if the assumption
there has to be at least one non empty link is just wrong. It certainly
does not hurt to add a small consistency check in either case.

Change-Id: I098446deef96a8baae26a7ca1ddd96e626a06dc5
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/693
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>