net: fec: Call swap_buffer() prior to IP header alignment

Commit 3ac72b7b ("net: fec: align IP header in hardware") breaks
networking on mx28.

There is an erratum on mx28 (ENGR121613 - ENET big endian mode
not compatible with ARM little endian) that requires an additional
byte-swap operation to workaround this problem.

So call swap_buffer() prior to performing the IP header alignment
to restore network functionality on mx28.

Fixes: 3ac72b7b

 ("net: fec: align IP header in hardware")
Reported-and-tested-by: Henri Roosen <henri.roosen@ginzinger.com>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>