sfxge: infer external port numbering for Pavia

Adjust external port mapping table to distinguish Pavia from Monza.
Now the presence of any 40G mode implies at least 2 outputs per
external port. So Pavia 4x10G ports are now mapped to 1,2,3,4;
Monza 4x10G ports map to 1,1,2,2 as before.

Submitted by: Richard Houldsworth <rhouldsworth at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days