mt76: usb: fix possible memory leak in mt76u_buf_free

Move q->ndesc initialization before the for loop in mt76u_alloc_rx
since otherwise allocated urbs will not be freed in mt76u_buf_free
Double-check scatterlist pointer in mt76u_buf_free

Fixes: b40b15e1 ("mt76: add usb support to mt76 layer")
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>