mt76x2: fix mrr idx/count estimation in mt76x2_mac_fill_tx_status()

Fix mcs and attempt count estimation in mt76x2_mac_fill_tx_status routine
if the number of tx retries reported by the hw is grater than
IEEE80211_TX_MAX_RATES

Fixes: 7bc04215 ("mt76: add driver code for MT76x2e")
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>