soc: xilinx: Fix error code in zynqmp_pm_probe()

This should be returning PTR_ERR() but it returns IS_ERR() instead.

Fixes: ffdbae28d9d1 ("drivers: soc: xilinx: Use mailbox IPI callback")
Signed-off-by: Dan Carpenter <[e-mail]>
Reviewed-by: Michal Simek <[e-mail]>
Signed-off-by: Michal Simek <[e-mail]>
Link: https://lore.kernel.org/r/20200605110020.GA978434@mwanda

drivers/soc/xilinx/zynqmp_power.c

···
 		rx_chan = mbox_request_channel_byname(client, "rx");
 		if (IS_ERR(rx_chan)) {
 			dev_err(&pdev->dev, "Failed to request rx channel\n");
-			return IS_ERR(rx_chan);
+			return PTR_ERR(rx_chan);
 		}
 	} else if (of_find_property(pdev->dev.of_node, "interrupts", NULL)) {
 		irq = platform_get_irq(pdev, 0);