sandbox: eth-raw-os: successful return code is 0

all network drivers return 0 on the successful
transmission.

Signed-off-by: Maxim Uvarov <maxim.uvarov@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>

arch/sandbox/cpu/eth-raw-os.c

@@ -256,7 +256,7 @@ int sandbox_eth_raw_os_send(void *packet, int length,
 		       strerror(errno));
 		return -errno;
 	}
-	return retval;
+	return 0;
 }
 
 int sandbox_eth_raw_os_recv(void *packet, int *length,