ACPI: dock: add struct dock_station * directly to platform device data

Instead of adding a (struct dock_station **) to our dock device's
platform data, we can add the (struct dock_station *) directly.

This change saves us some ugly casting and improves readability.

The cost of making this change is an extra 290 bytes of stack usage,
but this is an infrequently called code-path and unlikely to cause
the kernel to blow up.

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>