net/temac: FIX segfault when process old irqs

Do not enable the irq until the scatter gather registers are ready to
handle the data. Otherwise an irq from a packet send/received before
last close can lead to an access to an invalid memory region on the irq
handler.

Also, stop the dma engine on close.

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>