ACPI: hp-wmi, msi-wmi: clarify that wmi_install_notify_handler() returns an acpi_status

Emphasize that that wmi_install_notify_handler() returns an acpi_status
rather than -errno by by testing ACPI_SUCCESS(), ACPI_FAILURE().

No functional change in this patch, but this confusion caused a bug in dell-wmi.

Signed-off-by: Len Brown <len.brown@intel.com>

drivers/platform/x86/msi-wmi.c

@@ -236,7 +236,7 @@ static int __init msi_wmi_init(void)
 	}
 	err = wmi_install_notify_handler(MSIWMI_EVENT_GUID,
 			msi_wmi_notify, NULL);
-	if (err)
+	if (ACPI_FAILURE(err))
 		return -EINVAL;
 
 	err = msi_wmi_input_setup();