dwc_eth_qos: release descriptors outside netif_tx_lock

To prepare for using the CMA, we can not be in atomic context when
de-allocating DMA buffers.

The tx lock was needed only to protect the hw reset against the xmit
handler. Now we briefly grab the tx lock while stopping the queue to
make sure no thread is inside or will enter the xmit handler.

Signed-off-by: Lars Persson <larper@axis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>