ath9k: Fix incorrect access of rate flags in RC

The index variable to access the rate flags should be obtained from the
inner loop counter which corresponds to the rate table structure.This
fixes the invalid rate selection i.e when the supported basic rate is
invalid on a particular band and also the following warning message.
Thanks to Raj for finding this out.

Call Trace:

 [<ffffffff8104ee4a>] warn_slowpath_common+0x7a/0xb0

 [<ffffffff8104ee95>] warn_slowpath_null+0x15/0x20

 [<ffffffffa0583c45>] ath_get_rate+0x595/0x5b0 [ath9k]

 [<ffffffff811a0636>] ? cpumask_next_and+0x36/0x50

 [<ffffffffa0405186>] rate_control_get_rate+0x86/0x160 [mac80211]

 [<ffffffffa040dfac>] invoke_tx_handlers+0x81c/0x12d0 [mac80211]

 [<ffffffffa040eae9>] ieee80211_tx+0x89/0x2b0 [mac80211]

 [<ffffffff812891bc>] ? pskb_expand_head+0x1cc/0x1f0

 [<ffffffffa040edc5>] ieee80211_xmit+0xb5/0x1c0 [mac80211]

 [<ffffffffa041026f>] ieee80211_tx_skb+0x4f/0x60 [mac80211]

 [<ffffffffa03fe016>] ieee80211_send_nullfunc+0x46/0x60 [mac80211]

 [<ffffffffa03f91d7>] ieee80211_offchannel_stop_station+0x107/0x150
[mac80211]

 [<ffffffff812891bc>] ? pskb_expand_head+0x1cc/0x1f0

 [<ffffffffa040edc5>] ieee80211_xmit+0xb5/0x1c0 [mac80211]

 [<ffffffffa041026f>] ieee80211_tx_skb+0x4f/0x60 [mac80211]

 [<ffffffffa03fe016>] ieee80211_send_nullfunc+0x46/0x60 [mac80211]

 [<ffffffffa03f91d7>] ieee80211_offchannel_stop_station+0x107/0x150
[mac80211]

 [<ffffffffa03f8896>] ieee80211_scan_work+0x146/0x600 [mac80211]

 [<ffffffff8133a375>] ? schedule+0x2f5/0x8e0

 [<ffffffffa03f8750>] ? ieee80211_scan_work+0x0/0x600 [mac80211]

 [<ffffffff81064fcf>] process_one_work+0x10f/0x380

 [<ffffffff81066bc2>] worker_thread+0x162/0x340

 [<ffffffff81066a60>] ? worker_thread+0x0/0x340

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