net: fec: fix receive VLAN CTAG HW acceleration issue

The current driver support receive VLAN CTAG HW acceleration feature
(NETIF_F_HW_VLAN_CTAG_RX) through software simulation. There calls the
api .skb_copy_to_linear_data_offset() to skip the VLAN tag, but there
have overlap between the two memory data point range. The patch just fix
the issue.

V2:
Michael Grzeschik suggest to use memmove() instead of skb_copy_to_linear_data_offset().

Reported-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Fixes: 1b7bde6d

 ("net: fec: implement rx_copybreak to improve rx performance")
Signed-off-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>