lib: rsa: Fix const-correctness of rsassa_pss functions

Prior to introduction of modifications in rsassa_pss functions
related to padding verification, doing a pass to update
const-correctness in targeted functions to comply with
coding-rules and avoid const-cast

Signed-off-by: SESA644425 <gioja.hermann@non.se.com>
Reviewed-by: Simon Glass <sjg@chromium.org>