serial: Fix control reaching end of non-void function

Detected with CFLAGS="-Wall -Werror":

serial.c: In function ‘mctp_serial_rx’:
serial.c:325:1: error: control reaches end of non-void function [-Werror=return-type]
  325 | }
      | ^
cc1: all warnings being treated as errors

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: I59ee6c392c6172f3d978076df0278c8b73d69931
diff --git a/serial.c b/serial.c
index d59bf44..808c215 100644
--- a/serial.c
+++ b/serial.c
@@ -322,6 +322,7 @@
 		const void *buf, size_t len)
 {
 	mctp_rx_consume(serial, buf, len);
+	return 0;
 }
 
 static int mctp_serial_core_start(struct mctp_binding *binding)