ath6kl: add error message to explain lack of HT

It can take a user a while to understand why their
NIC that advertises 802.11n support cannot actually
do 802.11n.  Print out a warning in the logs to save
the next poor person to use this NIC some trouble.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>