firmware: raspberrypi: Use dev_err_probe() to simplify code

In the probe path, dev_err() can be replaced with dev_err_probe()
which will check if error code is -EPROBE_DEFER and prints the
error name. It also sets the defer probe reason which can be
checked later through debugfs.

Signed-off-by: Yuan Can <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Florian Fainelli <[e-mail]>

drivers/firmware/raspberrypi.c

···
 	fw->cl.tx_block = true;
 
 	fw->chan = mbox_request_channel(&fw->cl, 0);
-	if (IS_ERR(fw->chan)) {
-		int ret = PTR_ERR(fw->chan);
-		if (ret != -EPROBE_DEFER)
-			dev_err(dev, "Failed to get mbox channel: %d\n", ret);
-		return ret;
-	}
+	if (IS_ERR(fw->chan))
+		return dev_err_probe(dev, PTR_ERR(fw->chan),
+				     "Failed to get mbox channel\n");
 
 	init_completion(&fw->c);
 	kref_init(&fw->consumers);