drivers: ethernet: eth_nxp_enet_qos: fix rx buffer processing order

Always reading from descriptor with index 0 could cause processing
of the buffers in a different order than they were received. Fixed by
reading from the next unprocessed position in the ring of descriptors
instead.

Fixed unused variable warnings.

Signed-off-by: Stanislav Poboril <stanislav.poboril@nxp.com>