ACPI: dock: combine add|alloc_dock_dependent_device (v2)

There's no real need to have a separate allocation step when adding
a dock dependent device.

Combining the two functions is both logical and helps with legibility.

Signed-off-by: Alex Chiang <achiang@hp.com>
Acked-by: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>