ACPI / ACPICA: Avoid crashing if _PRW is defined for the root object

Some ACPI BIOSes define _PRW for the root object which causes
acpi_setup_gpe_for_wake() to crash when trying to dereference the
bogus device_node pointer.  Avoid the crash by checking if
wake_device is not the root object before attempting to set up the
"implicit notify" mechanism for it.

The problem was introduced by commit bba63a29
(ACPICA: Implicit notify support) that added the wake_device argument
to acpi_setup_gpe_for_wake().

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>