dpaa_eth: use only one buffer pool per interface

Currently the DPAA Ethernet driver is using three buffer pools
for each interface, with three different sizes for the buffers
provided for the FMan reception path. This patch reduces the
number of buffer pools to one per interface. This change is in
preparation of another, that will be switching from netdev_frags
to page backed buffers for the receive path.

Signed-off-by: Madalin Bucur <madalin.bucur@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>