cmd: clk: Check return value from soc_clk_dump

In case of error in soc_clk_dump function are returned different values
then CMD return values (-1, 0, 1).

For example:
ZynqMP> clk dump
exit not allowed from main input shel

The patch is checking all negative return values and return
CMD_RET_FAILURE which is proper reaction for these cases.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>

cmd/clk.c

@@ -16,7 +16,15 @@ int __weak soc_clk_dump(void)
 static int do_clk_dump(cmd_tbl_t *cmdtp, int flag, int argc,
 		       char *const argv[])
 {
-	return soc_clk_dump();
+	int ret;
+
+	ret = soc_clk_dump();
+	if (ret < 0) {
+		printf("Clock dump error %d\n", ret);
+		ret = CMD_RET_FAILURE;
+	}
+
+	return ret;
 }
 
 static cmd_tbl_t cmd_clk_sub[] = {