bnx2x: properly update skb when mtu > 1500

Since commit e52fcb24 newly allocated
skb for small packets are not updated properly and dropped by stack.

Signed-off-by: Dmitry Kravkov <dmitry@broadcom.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>