mt76: mt7915: fix rxrate reporting

Avoid directly updating sinfo->rxrate from firmware since rate_info might
be overwritten by wrong results even mt7915_mcu_get_rx_rate() fails check.

Add more error handlings accordingly.

Fixes: 11553d88

 ("mt76: mt7915: query station rx rate from firmware")
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>