kernel/ath: Make it compile into the kernel again and re-add to GENERIC.

kernel/ath: Make it compile into the kernel again and re-add to GENERIC.

Full list of changes:

* Adjust the 'files' file for changed ath_hal path and additional
  source.

* Add options from FreeBSD and comment those out which are not
  compiling/used. Add the remaining ones to the LINT kernels for
  additional build checking.

* Fix some typos in the ar9300 code.

* Use ath_hal_ether_sprintf() where needed.

* Add some casts to work around -Wenum-compare and -Wformat breakage.

* Comment out some unused struct ifp's and FreeBSD's ktr.

* Slightly readjust ATH_RXBUF and ATH_TXBUF handling to let the kernel
  options of the same name override the defaults. In the GENERIC kernel
  and module everything stays the same.

* Use the (slightly better structured) FreeBSD Makefile for ath_hal.

All kernels compile with ath(4) as of this commit.