ath9k: stop on rates with idx -1 in ath9k rate control's .tx_status

Rate control algorithms are supposed to stop processing when they
encounter a rate with the index -1.  Checking for rate->count not being
zero is not enough.

Allowing a rate with negative index leads to memory corruption in
ath_debug_stat_rc().

One consequence of the bug is discussed at
https://bugzilla.redhat.com/show_bug.cgi?id=768639

Signed-off-by: Pavel Roskin <proski@gnu.org>
Cc: stable@vger.kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>