dpaa2-eth: Minor refactoring in ethtool stats

As we prepare to read more pages from the DPNI stat counters,
reorganize the code a bit to make it easier to extend.

Signed-off-by: Ioana Radulescu <ruxandra.radulescu@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>