nixos/networkd: Fix eval error for defaultGateway

Regression introduced by 0cb487ee

.

This changed the result for defaultGateway to be a submodule instead of
just a plain string, so instead of using just cfg.defaultGateway we need
to pass cfg.defaultGateway.address now.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @abbradar