net: mvneta: mark mapped and tso buffers separately

Mark dma-mapped skbs and TSO buffers separately, so we can use
buf->type to identify their differences.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>