x86/srat: fix parsing of APIC IDs > MAX_APIC_ID

Ignore them like it's done in the MADT parser. This allows booting on a box
with SRAT and APIC IDs > 255.

Reported by: Wei Liu <wei.liu2@citrix.com>
Tested by: Wei Liu <wei.liu2@citrix.com>
Reviewed by: kib
MFC after: 2 weeks
Sponsored by: Citrix Systems R&D