net/enetc: improve batching Rx ring refill

net/enetc: improve batching Rx ring refill

Move from doing batch refill of Rx ring from bundles of 8 to once per
enetc_clean_rx_ring call.  One benefit is that we're cleaning up all the
BDs that we just processed, which should still be cached.  The other is
that hardware Rx index stays a little back and doesn't cause contention
on the BDs processed in the Rx loop.

Signed-off-by: Alex Marginean <alexandru.marginean@nxp.com>
Acked-by: Gagandeep Singh <g.singh@nxp.com>