Fix downgrade in migration

In method downgrade_cisco in migration folsom_initial instead of
local method drop_tables op.drop_tables is used

Closes-Bug: #1243148

Change-Id: Ida6c52653c065e49c85141a7d1e61d394c1d709e

neutron/db/migration/alembic_migrations/versions/folsom_initial.py

@@ -538,7 +538,7 @@ def downgrade_brocade():
 
 
 def downgrade_cisco():
-    op.drop_tables(
+    drop_tables(
         'nexusport_bindings',
         'port_bindings',
         'credentials',