mt76: mt7615: check sta_rates pointer in mt7615_sta_rate_tbl_update

Check sta_rates pointer value in mt7615_sta_rate_tbl_update routine
since minstrel_ht_update_rates can fail allocating rates array.

Fixes: 04b8e659 ("mt76: add mac80211 driver for MT7615 PCIe-based chipsets")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>