API: add mctp_set_tx_enabled

Provide a method for MCTP bindings to implement flow control

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
diff --git a/serial.c b/serial.c
index 2fa3abe..22c1b4b 100644
--- a/serial.c
+++ b/serial.c
@@ -288,6 +288,7 @@
 {
 	assert(serial->fd >= 0);
 	mctp_register_bus(mctp, &serial->binding, eid);
+	mctp_binding_set_tx_enabled(&serial->binding, true);
 }
 
 struct mctp_binding_serial *mctp_serial_init(void)