ovn: Fix check on existing encap row.

This code does some checking to validate the existing encaps for a
chassis to see if they need to be updated.  This typo resulted in
ovn-controller re-creating its encap(s) every time this code ran, making
ovn-controller and ovsdb-server eat up a CPU in my testing.

Signed-off-by: Russell Bryant <rbryant@redhat.com>
Acked-by: Ben Pfaff <blp@nicira.com>