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/libmctp.h b/libmctp.h
index 523891f..b36a943 100644
--- a/libmctp.h
+++ b/libmctp.h
@@ -97,6 +97,7 @@
 	struct mctp	*mctp;
 	int		pkt_size;
 	int		pkt_pad;
+	int		(*start)(struct mctp_binding *binding);
 	int		(*tx)(struct mctp_binding *binding,
 				struct mctp_pktbuf *pkt);
 };