ath9k: fix ath_get_rate_txpower() to respect the rate list end tag

Stop reading (and copying) from ieee80211_tx_rate to ath_tx_info.rates
after list end tag (count == 0, idx < 0), prevents copying of garbage
to card registers.

Note: no need to write to the remaining ath_tx_info.rates entries
as the complete ath_tx_info struct is already initialized to zero from
both call sites.

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20220402153014.31332-1-ps.report@gmx.net