cgtqmx6eval: README: Fix parameter of 'sf write' command

'sf command' should receive the length in sector boundaries, so fix it
accordingly.

Also, show the steps to set the server and board IP addresses for
making the steps clearer.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>