net: fec: fix regression on i.MX28 introduced by rx_copybreak support

commit 1b7bde6d ("net: fec: implement rx_copybreak to improve rx performance")
introduced a regression for i.MX28. The swap_buffer() function doing
the endian conversion of the received data on i.MX28 may access memory
beyond the actual packet size in the DMA buffer. fec_enet_copybreak()
does not copy those bytes, so that the last bytes of a packet may be
filled with invalid data after swapping.
This will likely lead to checksum errors on received packets.
E.g. when trying to mount an NFS rootfs:
UDP: bad checksum. From 192.168.1.225:111 to 192.168.100.73:44662 ulen 36

Do the byte swapping and copying to the new skb in one go if
necessary.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Tested-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>