mac80211: make Minstrel the default rate control algorithm

This makes minstrel the default rate control algorithm
for mac80211. For more information see:

http://wireless.kernel.org/en/developers/Documentation/mac80211/RateControl/minstrel



If someone can come up with a better algorithm they get a prize
(undisclosed).

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>