ath9k: fix max phy rate at rate control init

ath9k: fix max phy rate at rate control init

commit 10636bc2d60942254bda149827b922c41f4cb4af upstream.

The stations always chooses 1Mbps for all trasmitting frames,
whenever the AP is configured to lock the supported rates.
As the max phy rate is always set with the 4th from highest phy rate,
this assumption might be wrong if we have less than that. Fix that.

Cc: Paul Stewart <pstew@google.com>
Reported-by: Ajay Gummalla <agummalla@google.com>
Signed-off-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>