mgmt: mcumgr: drop few redeclared/shadowed variables

Drop few redeclared variables, fixes some:

settings_mgmt.c:454:30: error: declaration of ok shadows a previous
local [-Werror=shadow]
  454 |                         bool ok;
      |                              ^~
settings_mgmt.c:436:14: note: shadowed declaration is here
  436 |         bool ok = true;
      |              ^~

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>