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/common/command.c b/common/command.c
index 2c491e2..068cb55 100644
--- a/common/command.c
+++ b/common/command.c
@@ -475,13 +475,13 @@
 		case 'l':
 			return 4;
 		case 's':
-			return -2;
+			return CMD_DATA_SIZE_STR;
 		case 'q':
 			if (MEM_SUPPORT_64BIT_DATA)
 				return 8;
 			/* no break */
 		default:
-			return -1;
+			return CMD_DATA_SIZE_ERR;
 		}
 	}
 	return default_size;