Update the code to replace instances of snprintf and a pointer update with
just calling ethtool_sprintf.

Also replace the char pointer with a u8 pointer to avoid having to recast
the pointer type.
Acked-by: NMichael S. Tsirkin <mst@redhat.com>
Acked-by: NJason Wang <jasowang@redhat.com>
Signed-off-by: NAlexander Duyck <alexanderduyck@fb.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>