cmd/io.c: Fix comparison of unsigned expression warning

The function cmd_get_data_size() returns an int and not unsigned.  So we
should assign it to an int rather than unsigned so that we can later
compare the return value.  Reported by clang-3.8.

Signed-off-by: Tom Rini <trini@konsulko.com>

cmd/io.c

@@ -46,7 +46,8 @@ int do_io_iod(cmd_tbl_t *cmdtp, int flag, int argc, char *const argv[])
 
 int do_io_iow(cmd_tbl_t *cmdtp, int flag, int argc, char *const argv[])
 {
-	ulong addr, size, val;
+	ulong addr, val;
+	int size;
 
 	if (argc != 3)
 		return CMD_RET_USAGE;