[FIX] website_blog: fix size options values when blog regular cover

Before this commit, the widget value was updated (based on the current
state) prior to updating the snippet UI. As a result, the widget value
reflected used the non-updated UI labels.

task-3112890

closes odoo/odoo#120569

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>