command: Add constants for cmd_get_data_size string / error

At present these values are open-coded in a few places. Add constants so
the meaning is clear.

Also add a comment to cmd_get_data_size()

Signed-off-by: Simon Glass <sjg@chromium.org>

cmd/mem.c

@@ -393,7 +393,7 @@ static int do_mem_search(struct cmd_tbl *cmdtp, int flag, int argc,
 		 * Defaults to long if no or incorrect specification.
 		 */
 		size = cmd_get_data_size(argv[0], 4);
-		if (size < 0 && size != -2 /* string */)
+		if (size < 0 && size != CMD_DATA_SIZE_STR)
 			return 1;
 
 		argc--;