mt76: mt7921: fix possible AOOB issue in mt7921_mcu_tx_rate_report

Fix possible array out of bound access in mt7921_mcu_tx_rate_report.
Remove unnecessary varibable in mt7921_mcu_tx_rate_report

Fixes: 1c099ab4

 ("mt76: mt7921: add MCU support")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/91a1e8f6b6a3e6a929de560ed68132f6eb421720.1619187875.git.lorenzo@kernel.org