Ath6kl: Initialize hardware parameters.

* Added a bunch of defines to core.h for the various hardware;
* Added struct ath6kl_hw to struct ath6kl_softc;
* Added new files and update Makefile accordingly:
    o init.c (only ath6kl_init_hw_params enabled)
    o target.h;
* Call ath6kl_init_hw_params from ath6kl_core_init;