Introduce ieee80211_min_basic_rate() and ieee80211_max_basic_rate().

Introduce ieee80211_min_basic_rate() and ieee80211_max_basic_rate().

These helpers can be used by drivers to improve compatibility with APs
that disable some mandatory PHY rates in the basic rate set.

For instance, many of our drivers hard-code 11b rates on 2 Ghz and run
into problems when APs disable them. Since 11b rates are being disabled
by default by some vendors, hardcoding them is not a good idea anymore.

ok mpi@ phessler@