Fix possible NULL pointer dereference in mt7915_mac_fill_rx_vector
routine if the chip does not support dbdc and the hw reports band_idx
set to 1.

Fixes: 78fc30a2 ("mt76: mt7915: move testmode data from dev to phy")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>