ACPI / dock: Actually define acpi_dock_init() as void

Commit 94add0f8 (ACPI / dock: Initialize ACPI dock subsystem upfront)
changed the header of acpi_dock_init() in internal.h so that it is
supposed to be a void function now, but it forgot to update its
actual definition in dock.c according to which it still is supposed
to return int.

Although that didn't cause any visible breakage or even a compiler
warning to be thrown, which is odd enough, fix it.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: 3.10+ <stable@vger.kernel.org>