ACPI: Fix CONFIG_ACPI_DOCK=n compiler warning

Recently the ACPI ops structs were constified but the inline version
of register_hotplug_dock_device() was overlooked (see also commit
9c8b04be, June 25 2011). Update the inline function
register_hotplug_dock_device() that is enabled with
CONFIG_ACPI_DOCK=n too. This patch fixes at least the following
compiler warnings:

drivers/ata/libata-acpi.c: In function .ata_acpi_associate.:
drivers/ata/libata-acpi.c:266:11: warning: passing argument 2 of .register_hotplug_dock_device. discards qualifiers from pointer target type
include/acpi/acpi_drivers.h:146:19: note: expected .struct acpi_dock_ops *. but argument is of type .const struct acpi_dock_ops *.
drivers/ata/libata-acpi.c:275:11: warning: passing argument 2 of .register_hotplug_dock_device. discards qualifiers from pointer target type
include/acpi/acpi_drivers.h:146:19: note: expected .struct acpi_dock_ops *. but argument is of type .const struct acpi_dock_ops *.

Cc: stable@vger.kernel.org
Signed-off-by: Len Brown <len.brown@intel.com>