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>