ath9k: Fix error reported by smatch

debug_sta.c: ath_debug_rate_stats() error: buffer overflow 'rstats->ht_stats' 24 <= 24

Cc: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>