mt76: mt76u: fix typo in mt76u_fill_rx_sg

Fix typo setting urb->transfer_buffer_length in mt76u_fill_rx_sg

Fixes: b40b15e1 ("mt76: add usb support to mt76 layer")
Fixes: f8f527b1

 ("mt76: usb: use EP max packet aligned buffer sizes for rx")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>