net: mvneta: move tso_build_hdr() into mvneta_tso_put_hdr()

Move tso_build_hdr() into mvneta_tso_put_hdr() so that all the TSO
header building code is in one place.

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>