mailbox: mailbox-test: Fix an error check in mbox_test_probe()

mbox_test_request_channel() function returns NULL or
error value embedded in the pointer (PTR_ERR).
Evaluate the return value using IS_ERR_OR_NULL.

Signed-off-by: Minjie Du <duminjie@vivo.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

Author: Minjie Du

drivers/mailbox/mailbox-test.c

@@ -388,7 +388,7 @@ static int mbox_test_probe(struct platform_device *pdev)
 	tdev->tx_channel = mbox_test_request_channel(pdev, "tx");
 	tdev->rx_channel = mbox_test_request_channel(pdev, "rx");
 
-	if (!tdev->tx_channel && !tdev->rx_channel)
+	if (IS_ERR_OR_NULL(tdev->tx_channel) && IS_ERR_OR_NULL(tdev->rx_channel))
 		return -EPROBE_DEFER;
 
 	/* If Rx is not specified but has Rx MMIO, then Rx = Tx */