This brings the AHB support in line with the PCI support - now other "things"
can wrap up the calibration / board data into a firmware blob and have them
probe/attach after the system has finished booting.
Note that this change requires /all/ of the AHB using kernel configurations
to change - so until I drop those changes in, this breaks AHB.
Fear not, I'll do that soon.
TODO:
* the above stuff.
Tested:
* AR9331, carambola 2, loading if_ath / wlan as modules at run time
7a650e6a6a