net_interface & datalink: Fix MTU handling.

The device is what actually controls the MTU, and it has its own
field for this, so having a second one just meant the MTU never
got updated after startup.

Remove the "mtu" field from the interface, use the "device->mtu" directly,
and then actually invoke device->module->set_mtu when updating.

headers/private/net/net_datalink.h

@@ -48,7 +48,6 @@ typedef struct net_interface {
 	uint32				index;
 	uint32				flags;
 	uint8				type;
-	uint32				mtu;
 	uint32				metric;
 } net_interface;