fec: Use NAPI_POLL_WEIGHT

Instead of using a custom 'FEC_NAPI_WEIGHT', just use the generic
'NAPI_POLL_WEIGHT' definition instead.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>