net: stmmac: use correct pointer when printing normal descriptor ring

There are two pointers in sysfs_display_ring,
one that increments if using normal dma descriptors,
another if using extended dma descriptors.

When printing the normal dma descriptors, the wrong pointer is used,
thus the printed descriptor addresses are incorrect.

Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>