mt76: usb: use a linear buffer for tx/rx datapath if sg is not supported

Use linear fragment and not a single usb scatter-gather buffer in mt76u
{tx,rx} datapath if the usb controller has sg data length constraints

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>