[settings] fix bootloop when more than 255 ChildInfo entries are stored (#4640)

In MleRouter::RestoreChildren() there is an iteration over all
ChildInfo items. This iterator uses mIndex which is uint8_t and in our
case its value wraps causing infinite loop. Changing mIndex to
uint16_t fixes this issue.

src/core/common/settings.hpp

@@ -763,7 +763,7 @@ public:
         void Read(void);
 
         ChildInfo mChildInfo;
-        uint8_t   mIndex;
+        uint16_t  mIndex;
         bool      mIsDone;
     };