mv643xx_eth: fix receive checksumming

We have to explicitly tell the hardware to include the pseudo-header
when doing receive checksumming, otherwise hardware checksumming will
fail for every received packet and we'll end up setting CHECKSUM_NONE
on every received packet.

While we're at it, when skb->ip_summed is set to CHECKSUM_UNNECESSARY
on received packets, skb->csum is supposed to be undefined, and thus
there is no need to set it.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>