Interworking: Fix last-network preference to not override priority

Interworking: Fix last-network preference to not override priority

Commit 3d910ef497b11e149cf41e772670f7a7fe3a1e19 tried to make
last-network selection behave more consistently with Interworking
network selection preferences. However, it did not take into account
that other network block may have higher priority. In such cases, the
last added network from Interworking network selection should actually
not be selected for the next connection. Fix this by limiting the
last-network preference to work only within a priority class.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>