[ath_hal_ar9300] Add some comments around the AR9300 ANI code.

I'm refamiliarising myself with the behaviour of the ANI code and I thought
I'd drop some comments to remind myself.

[ath_hal_ar9300] Add the missing bits from the previous HAL commit.

Noticed by: 75+ emails telling me I messed up.

[ath_hal] Return failure if noise floor calibration fails.

If we fail noise floor calibration then we may end up with a deaf NIC
which we can't recover without a full chip reset.

Earlier chips seem to get less stuck in this condition versus AR9280/later
and AR9300/later, but whilst here just fix up the AR5212 era chips to also
return NF calibration failures.

This HAL routine would only return failure if the channel was not configured.

This is a no-op until the driver side code for doing resets and the HAL
code for being told about the reset type (and then handling it!) is
implemented.

Tested:

* AR9280, STA mode
* AR2425, STA mode
* AR9380, STA mode

Fix compilation error on gcc-5.2.0 - it now warns on non-paranthen'ed
logical negation when used in this fashion.

Tested:

* compile only

Expose the ANI state / statistics using the public ANI function, rather than
the ar9300_* definitions.

.. which of course don't match, and athstats was reading garbage ANI
data.

Add initial support for the HAL channel survey support to the AR9300 HAL.

This is used by the 'athsurvey' command to print out channel survey
statistics - % busy times transmit, receive and airtime.

It's as buggy and incomplete as the rest of the HAL survey support -
notably, tying into the ANI code to read channel stats and occasionally
getting garbage counters isn't very nice. It also doesn't (yet!) get
channel survey information during a scan. But it's good enough for
basic air-time debugging, which is why I'm committing it in this state.

Tested:

* AR9380, STA mode

Skip the OFDM weak signal threshold detection programming for
AR9462/AR9565.

This and some upcoming changes to the HAL for these chips should
address some of the signal sensitivity reported by users.

Tested:

* AR9462 (WB222), STA mode

Obtained from: Linux ath9k

Sync from git - ah_config is in 'ath_hal', not 'ath_hal_private' on
FreeBSD.

Bring over my FreeBSD modifications for the AR9300 HAL to make it
work in FreeBSD.

This is still heavily a work in progress but I'd rather it start
shipping in -HEAD sooner rather than later.

This doesn't (yet) link it into the build system either for a static
kernel or as a module; that will come later (after many, many make universe
tests.)