net: mv643xx_eth: Factorize initial checksum and command preparation

Make the code more readable by moving the initial checksum setup
and the command/status preparation to its own function.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
net: mv643xx_eth: Implement software TSO

Now that the TSO helper API has been introduced, this commit makes use
of it to implement the TSO in this driver.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>