ACPI / sleep: clean up compiler warning about uninitialized field

Initialize pwr_btn_status as disabled which (a) makes sure it is
in a proper state to start, and (b) cleans up a compiler warning
about an uninitialized variable.

Signed-off-by: Al Stone <al.stone@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>