Fix device name assignment for SystemACE (from "xs`" to "xsa").

This fixes a bug introduced in 5d10302f, where device trees that don't
provide the "port-number" attribute are mistakenly assigned the device "xs`".
The error check that's supposed to assign a default letter can't succeed,
since it tests an unsigned type against a negative return code.

Signed-off-by: Graeme Smecher <gsmecher@threespeedlogic.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>