usb: gadget: u_ether: synchronize with transmit when stopping queue

When disconnecting, it's possible that another thread has already made it
into eth_start_xmit before we call netif_stop_queue. This can lead to a
crash as eth_start_xmit tries to use resources that gether_disconnect is
freeing. Use netif_tx_lock/unlock around netif_stop_queue to ensure no
threads are executing during the remainder of gether_disconnect.

Signed-off-by: Jeff Westfahl <jeff.westfahl@ni.com>
Tested-by: Jaeden Amero <jaeden.amero@ni.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>