net: enetc: remove forward-declarations of enetc_clean_{rx,tx}_ring

This patch moves the NAPI enetc_poll after enetc_clean_rx_ring such that
we can delete the forward declarations.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>