ACPI / dock: Drop redundant acpi_disabled check

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

acpi_init();
	if (acpi_disabled) return;
	acpi_scan_init();
		acpi_dock_init();
			if (acpi_disabled) /* redundant */
				return;

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