mgmt: mcumgr: stat: Fix stat_mgmt_list behavior

The MCUmgr statistics only work correctly when selecting
MCUMGR_SMP_LEGACY_RC_BEHAVIOUR option. Checks the option
flag on the stat_mgmt_list and fix the behavior.

Fixes: 80476

Signed-off-by: Gerson Fernando Budke <gerson.budke@ossystems.com.br>