net: sh_eth: fix skb_over_panic happen

When this GETHER controller received a large frame (about 1800 bytes
or more), skb_over_panic() happened. This is because the previous
driver set the RFLR to 0x1000 (4096 bytes) and the skb allocate size
is smaller than 4096 bytes. So, the controller accepted such a frame.

The controller can discard a large frame by the RFLR setting.
So, the patch modifies the value of RFLR to mtu + ETH_HLEN +
VLAN_HLEN + ETH_FCS_LEN.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>