isp(4): Allow more than 2 ports to read WWNs from NVRAM.

It appears at least on QLE2694L cards 3rd and 4th ports follow the
same NVRAM addressing logic as the first two. In lack of proper
documentation this guess is as good as it can be.

MFC after: 1 week
Sponsored by: iXsystems, Inc.

(cherry picked from commit 483e464ed4325a0710485925ecfbe0e1c8d6bb02)