Check sta_rates pointer value in mt7603_sta_rate_tbl_update routine
since minstrel_ht_update_rates can fail allocating rates array.

Fixes: c8846e10 ("mt76: add driver for MT7603E and MT7628/7688")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>