[ADD] hw_posbox_homepage: adapted Six fields

This PR adds the verification for user input fields related to Six
terminals on the IoT box homepage.
Since we are moving Six to IoT it can be dangerous to call C functions
with wrong arguments (this can lead to leaks, errors and crashes)

task-3031744

closes odoo/odoo#135667

Related: odoo/enterprise#47086
Signed-off-by: Yaroslav Soroko (yaso) <yaso@odoo.com>