modules: dataman: Optimize memory usage

Use the size of each item type instead of the biggest one.

In AeroFC that runs is constrained mode it was using 7860 bytes
and now it uses 6930 bytes almost 1KB less.

src/systemcmds/tests/test_dataman.c

@@ -63,6 +63,8 @@ int test_dataman(int argc, char *argv[]);
 
 #define NUM_MISSIONS_TEST 50
 
+#define DM_MAX_DATA_SIZE sizeof(struct mission_s)
+
 static int
 task_main(int argc, char *argv[])
 {