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