net: ll_temac: Handle DMA halt condition caused by buffer underrun

The SDMA engine used by TEMAC halts operation when it has finished
processing of the last buffer descriptor in the buffer ring.
Unfortunately, no interrupt event is generated when this happens,
so we need to setup another mechanism to make sure DMA operation is
restarted when enough buffers have been added to the ring.

Fixes: 92744989 ("net: add Xilinx ll_temac device driver")
Signed-off-by: Esben Haabendal <esben@geanix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>