net-next: ax88796: Fix MAC address reading

To read the MAC address from the (virtual) SAprom, the remote DMA
unit needs to be set up like for every other process access to card-local
memory.

Signed-off-by: Michael Karcher <kernel@mkarcher.dialup.fu-berlin.de>
Signed-off-by: Michael Schmitz <schmitzmic@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>