rtems-fdt/rtems-fdt.c: Fix bug in loop termination

The while loop, loops infinitely in case of raw FDT data.
The loop condition (size) is not modified during iterations.

Fixes #4350

cpukit/libmisc/rtems-fdt/rtems-fdt.c

@@ -582,7 +582,7 @@ rtems_fdt_load (const char* filename, rtems_fdt_handle* handle)
         close (bf);
         return -RTEMS_FDT_ERR_READ_FAIL;
       }
-      r -= size;
+      size -= r;
       buf += r;
     }
   }