platform/x86: use u8 * for raw byte buffers

platform/x86: use u8 * for raw byte buffers

The buffer parameters in populate_{security,string}_buffer() and
call_{biosattributes,password}_interface() are raw byte buffers, not
character strings - use 'u8 *' instead of 'char *' to reflect this.

Update the local buffer variables at the call sites and in
populate_security_buffer() accordingly.

Suggested-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Link: https://patch.msgid.link/20260410091633.2822-4-thorsten.blum@linux.dev
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>