Tweak the heuristic net80211 RA is using to decide whether enough

Tweak the heuristic net80211 RA is using to decide whether enough
statistics have been gathered for a candidate Tx rate. The goal is
to avoid Tx rate choices that might turn out to be too optimistic.

In practice this only affects the case where we probe upwards. If the
current Tx rate starts seeing loss we will still scale down very quickly.

Based on a larger collection of patches by Christian Ehrhardt.
I have made stylistic tweaks for consistency.

Tested:
iwn 6205: stsp, Josh Grosse
iwm 7265: stsp
iwm 8265: Matthias Schmidt
iwm 9260: phessler