API: Add binding accessors to generic struct mctp_binding

Rather than have every binding include their own wrappers around
binding<->core functions, introduce an accessor to retrieve the struct
mctp_binding from each.

This means we no longer need the binding-specific registration
callbacks. However, we do now need a ->start callback, to allow bindings
to perform post-registration init.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Change-Id: I6cee9e93f37520f85c155a0ca34017cc0675552c
diff --git a/utils/mctp-demux-daemon.c b/utils/mctp-demux-daemon.c
index f64f029..96d9753 100644
--- a/utils/mctp-demux-daemon.c
+++ b/utils/mctp-demux-daemon.c
@@ -145,7 +145,7 @@
 	if (rc)
 		return -1;
 
-	mctp_serial_register_bus(serial, mctp, eid);
+	mctp_register_bus(mctp, mctp_binding_serial_core(serial), eid);
 
 	binding->data = serial;
 
@@ -179,7 +179,8 @@
 		return -1;
 	}
 
-	mctp_astlpc_register_bus(astlpc, mctp, eid);
+	mctp_register_bus(mctp, mctp_binding_astlpc_core(astlpc), eid);
+
 	binding->data = astlpc;
 	return 0;
 }