net: ll_temac: Enable DMA when ready, not before

As soon as TAILDESCR_PTR is written, DMA transfers might start.
Let's ensure we are ready to receive DMA IRQ's before doing that.

Signed-off-by: Esben Haabendal <esben@geanix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>