ath5k: Fix survey

ath5k: Fix survey

The old survey implementation was broken and returned nonsense data.

Clear cycle counters and survey data on reset. Since the cycle counters easily
overflow it's better to keep a local version of collected survey data (in ms
resolution, instead of clockrate) and update this every time survey is
retrieved. If survey is retrieved often enough to avoid cycle counter overflows
this works fine, otherwise we could update survey more often, like ath9k does.
Still only the survey for the current channel is kept.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>