Changes

ss7_as: Optimize ss7_as_asp_assoc_find()

Look for counterpart on the object with the shortest list, ie. convert
from O(N) to O(min(N,M)).
This way eg. if we have 100 ASPs on 1 AS, lookup time becomes O(1).
Same if we have eg. 1 ASP serving 100 AS.

Change-Id: I139aede15af6b6a77d19e6fcf6b6abe8ed6599a6