ath9k: Fix a corner case failure in sending PAPRD frames in HT40

when the AP is configured with HT40/2.4GHz and when PAPRD is enabled in station
we have PAPRD training frames never sent out because of the following
failure "PAPRD target power delta out of range". This consistently happens
in channel 10,11 in Over The Air testing and rarely in channel 6 under wired
setup.
        the reason behind this issue is all the HT40 rates target power
are 0 when we operate in channel 10/11 at 2.4GHz

Signed-off-by: Mohammed Shafi Shajakhan <mshajakhan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>