net: broadcom: bcm4908_enet: fix received skb length

Use ETH_FCS_LEN instead of magic value and drop incorrect + 2

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>