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>
diff --git a/drivers/platform/x86/hp-wmi.c b/drivers/platform/x86/hp-wmi.c
index 8781d8fa..18bf741 100644
--- a/drivers/platform/x86/hp-wmi.c
+++ b/drivers/platform/x86/hp-wmi.c
@@ -581,7 +581,7 @@
 	if (wmi_has_guid(HPWMI_EVENT_GUID)) {
 		err = wmi_install_notify_handler(HPWMI_EVENT_GUID,
 						 hp_wmi_notify, NULL);
-		if (!err)
+		if (ACPI_SUCCESS(err))
 			hp_wmi_input_setup();
 	}