Re-introduce a global ath_hal_debug again for now, whilst I figure out what
to do about the few cases where the HAL state isn't available (regdomain)
or isn't yet setup (probe/attach.)

The global ath_hal_debug now affects all instances of the HAL.

This also restores the ability for probe/attach debugging to work; as
the sysctl tree may not be attached at that point. Users can just set
the global "hw.ath.hal.debug" to a suitable value to enable probe/attach
related debugging.