ACPI: Do not pass NULL to acpi_get_handle() when looking for _EJD

When trying to get the acpi_handle from an acpi_buffer, pass
ACPI_ROOT_OBJECT instead of NULL to acpi_get_handle(). This fixes the
detection of dock dependent bays.

Signed-off-by: Holger Macht <hmacht@suse.de>
Tested-by: Thomas Renninger <trenn@suse.de>
Signed-off-by: Len Brown <len.brown@intel.com>