ath9k: fix retry chain initialization in tx99 code

Initialize first chain attempt counter to 1 in ath9k_build_tx99_skb().
Otherwise multi-retry chain is initialized to {idx,count} = {-1, 0} in
rate_control_fill_sta_table() and tx99 transmission rate is not configured in
rate_control_apply_mask() since first chain idx is set to -1

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi83@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>