mt76: mt7915: fix txrate reporting

mt76: mt7915: fix txrate reporting

Properly check rate_info to fix unexpected reporting.

[ 1215.161863] Call trace:
[ 1215.164307]  cfg80211_calculate_bitrate+0x124/0x200 [cfg80211]
[ 1215.170139]  ieee80211s_update_metric+0x80/0xc0 [mac80211]
[ 1215.175624]  ieee80211_tx_status_ext+0x508/0x838 [mac80211]
[ 1215.181190]  mt7915_mcu_get_rx_rate+0x28c/0x8d0 [mt7915e]
[ 1215.186580]  mt7915_mac_tx_free+0x324/0x7c0 [mt7915e]
[ 1215.191623]  mt7915_queue_rx_skb+0xa8/0xd0 [mt7915e]
[ 1215.196582]  mt76_dma_cleanup+0x7b0/0x11d0 [mt76]
[ 1215.201276]  __napi_poll+0x38/0xf8
[ 1215.204668]  napi_workfn+0x40/0x80
[ 1215.208062]  process_one_work+0x1fc/0x390
[ 1215.212062]  worker_thread+0x48/0x4d0
[ 1215.215715]  kthread+0x120/0x128
[ 1215.218935]  ret_from_fork+0x10/0x1c

Fixes: e57b7901469f ("mt76: add mac80211 driver for MT7915 PCIe-based chipsets")
Fixes: e4c5ead632ff ("mt76: mt7915: rename mt7915_mcu_get_rate_info to mt7915_mcu_get_tx_rate")
Reported-by: Evelyn Tsai <evelyn.tsai@mediatek.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>