ACPICA: Events: add a return on failure from acpi_hw_register_read

This ensures that acpi_ev_fixed_event_detect() does not use fixed_status
and and fixed_enable as uninitialized variables.

Signed-off-by: Erik Schmauss <erik.schmauss@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>