net: lantiq: Wake TX queue again

The call to netif_wake_queue() when the TX descriptors were freed was
missing. When there are no TX buffers available the TX queue will be
stopped, but it was not started again when they are available again,
this is fixed in this patch.

Fixes: fe1a5642 ("net: lantiq: Add Lantiq / Intel VRX200 Ethernet driver")
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: David S. Miller <davem@davemloft.net>