enetc: Clean up Rx BD iteration

Improve maintainability of the code iterating the Rx buffer
descriptors to prepare it to support iterating extended Rx BD
descriptors as well.
Don't increment by one the h/w descriptor pointers explicitly,
provide an iterator that takes care of the h/w details.

Signed-off-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>