minstrel_ht: fixed rate mode through debugfs

Found several threads about fixed rate mode in minstrel_ht for test
environments, but no patches for it.

This patch provides such a mode through debugfs.

Signed-off-by: John W. Linville <linville@tuxdriver.com>

diff --cc net/mac80211/rc80211_minstrel.c
Simple merge

diff --cc net/mac80211/rc80211_minstrel.h
Simple merge

diff --cc net/mac80211/rc80211_minstrel_ht.c
Simple merge