Take into account possible error conditions of mt76_rx_token_consume
routine in mt7915_mmio_wed_init_rx_buf() and mt76_dma_add_buf()

Fixes: cd372b8c ("wifi: mt76: add WED RX support to mt76_dma_{add,get}_buf")
Fixes: 4f831d18 ("wifi: mt76: mt7915: enable WED RX support")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>