gif(4): Assert that gif_output() isn't called for EtherIP

With the current implementation of if_bridge(4), bridge_enqueue()
calls gif_transmit() only. Ensure it stays that way so that the
expectations in both drivers are either met or changed accordingly.

PR: 227450
(cherry picked from commit 8a0308722372a3ac6a94967ae4fe3123842309e2)