Improve the new ieee80211_{min,max}_rates() APIs and fix regressions.

Improve the new ieee80211_{min,max}_rates() APIs and fix regressions.

Instead of returning an index into ni_rates, return the RVAL of the
basic rate we want to use. This allows a driver to unambiguously map
the basic rate to the corresponding hardware-specific rate value, and
reduces the possibility of bugs where indices are used with arrays
they weren't intended for.

Adjust iwn(4) accordingly, and use the lowest instead of the highest
basic rate in iwn_tx() to cope better in noisy environments.

Fixes association problems on 5GHz reported by tb@