mv643xx_eth: fix NAPI weight being > 64

3.10-rc1 issues the following warning:

netif_napi_add() called with weight 128 on device eth%d

This patch reduce the weight to 64, using NAPI_POLL_WEIGHT.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>