dpaa_eth: add dropped frames to percpu ethtool stats

Prior to this change, the frames dropped on receive or transmit
were not displayed in the ethtool statistics, leaving the dropped
frames unaccounted for.

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