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>
diff --git a/cmd/itest.c b/cmd/itest.c
index a0cf4be..9a441ce 100644
--- a/cmd/itest.c
+++ b/cmd/itest.c
@@ -197,10 +197,10 @@
 #endif
 		value = binary_test (argv[2], argv[1], argv[3], w);
 		break;
-	case -2:
+	case CMD_DATA_SIZE_STR:
 		value = binary_test (argv[2], argv[1], argv[3], 0);
 		break;
-	case -1:
+	case CMD_DATA_SIZE_ERR:
 	default:
 		puts("Invalid data width specifier\n");
 		value = 0;