mt76: usb: fix warning in mt76u_buf_free

Fix following static checker warning in mt76u_buf_free:
  drivers/net/wireless/mediatek/mt76/usb.c:372 mt76u_buf_free()
  warn: address of 'urb->sg[i]' is non-NULL

There is no need to check sg page pointer since urb->num_sgs
has been already validated in mt76u_fill_rx_sg

Fixes: cb83585e ("mt76: usb: fix possible memory leak in mt76u_buf_free")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>