net: atlantic: add XPN handling

This patch adds XPN handling.
Our driver doesn't support XPN, but we should still update a couple
of places in the code, because the size of 'next_pn' field has
changed.

Signed-off-by: Mark Starovoytov <mstarovoitov@marvell.com>
Signed-off-by: Igor Russkikh <irusskikh@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>