ACPI: thinkpad-acpi: improve dock subdriver initialization

The dock sub-driver has split-personality (two subdrivers), and it was
doing some unoptimal things on init because of that.  Fix it so that the
second half of it will only init when necessary, and only if the first half
initialized sucessfully in the first place.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Len Brown <len.brown@intel.com>