usb: chipidea: debug: check before accessing ci_role

ci_role BUGs when the role is >= CI_ROLE_END.

Cc: stable@vger.kernel.org  #v3.10+
Signed-off-by: Michael Thalmeier <michael.thalmeier@hale.at>
Signed-off-by: Peter Chen <peter.chen@nxp.com>

drivers/usb/chipidea/debug.c

@@ -294,7 +294,8 @@ static int ci_role_show(struct seq_file *s, void *data)
 {
 	struct ci_hdrc *ci = s->private;
 
-	seq_printf(s, "%s\n", ci_role(ci)->name);
+	if (ci->role != CI_ROLE_END)
+		seq_printf(s, "%s\n", ci_role(ci)->name);
 
 	return 0;
 }