soc: xilinx: Fix error code in zynqmp_pm_probe()

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

Fixes: ffdbae28

 ("drivers: soc: xilinx: Use mailbox IPI callback")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/20200605110020.GA978434@mwanda