yk: Clean-up interface

* Use a void* for m_yk and cast it within.  Elimiantes ifdef mess for
  header file.
* Revise getSerial() so that the serial isn't cached as the previous
  implementation wouldn't scale with more then one physical key
  attached.  Intent is to overload getSerial(serial, dev).
yk: Remove instance() wrapper

* Remove nearly pointless wrapper
yk: Remove misleading comment

* Once upon a time this was true.