ethernet_interface: IPv6 Gateway is dynamic based on RA

The previous logic is technically incorrect, the gateway does not come
from DHCPv6 as the protocol is not intended to communicate the gateway.
Instead, this value is dynamic based on RA.

Change-Id: I1167204efaed422ef616250f34d181bc29666a75
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/src/ethernet_interface.cpp b/src/ethernet_interface.cpp
index fdd01c2..2851a1b 100644
--- a/src/ethernet_interface.cpp
+++ b/src/ethernet_interface.cpp
@@ -719,7 +719,7 @@
                 }
             }
 
-            if (!dhcp6())
+            if (!ipv6AcceptRA())
             {
                 auto gateway6 = EthernetInterfaceIntf::defaultGateway6();
                 if (!gateway6.empty())