wifi: mt76: mt7915: fix memory leak in mt7915_mmio_wed_init_rx_buf

Free mt76_txwi_cache pointer in mt7915_mmio_wed_init_rx_buf routine in
case of failure.

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>