ACPI: fix ATA_ACPI build

drivers/ata/libata-acpi.c fails to build

drivers/built-in.o: In function `ata_acpi_associate':
(.text+0x7106a): undefined reference to `register_hotplug_dock_device'

When CONFIG_ATA_ACPI=y and CONFIG_ACPI_DOCK=m
But if dock is selected from ata_acpi, dock will =y
when ata_acpi=y, avoiding this problem.

http://bugzilla.kernel.org/show_bug.cgi?id=10272

Signed-off-by: Len Brown <len.brown@intel.com>