ACPI / sleep: Drop redundant acpi_disabled check

acpi_sleep_init() is only called from acpi_bus_init() and the
code logic shows that it doesn't need to check acpi_disabled:

acpi_init();
	if (acpi_disabled) return;
	acpi_bus_init();
		acpi_sleep_init();
			if (acpi_disabled)
				return 0;

Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
[rjw: Subject and changelog]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>