Move the HAL channel survey support out to be in the top-level HAL,
rathe than private in each HAL module.

Whilst here, modify ath_hal_private to always have the per-channel
noisefloor stats, rather than conditionally. This just makes
life easier in general (no strange ABI differences between different
HAL compile options.)

Add a couple of methods (clear/reset, add) rather than using
hand-rolled versions of things.