mac80211: Warn users if HT fails because of freq mismatch.

I have a netgear WNDR3700 that appears to have an off-by-four
bug in how it fills out the hti->control_chan (I configure the
AP to channel 11, it reports 15 as control_chan).

Poke a message into the kernel logs to give users a
clue as to why they are not getting the expected
channel-type or rate.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>