cmd: part: Correct error handling

As 'part_get_info_by_name' now returns more status codes than just
-1 to indicate failure, we need to update the return value check.
Signed-off-by: NAnders Dellien <anders.dellien@arm.com>
Reviewed-by: NSean Anderson <sean.anderson@seco.com>

cmd/part.c

@@ -140,7 +140,7 @@ static int do_part_info(int argc, char *const argv[], enum cmd_part_info param)
 			return 1;
 	} else {
 		part = part_get_info_by_name(desc, argv[2], &info);
-		if (part == -1)
+		if (part < 0)
 			return 1;
 	}