pinctrl: fix missing unlock on error in pinctrl_groups_show()

Add the missing unlock on the error handle path in function
pinctrl_groups_show().

Cc: stable@kernel.org
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/pinctrl/core.c

@@ -1061,8 +1061,10 @@ static int pinctrl_groups_show(struct seq_file *s, void *what)
 			seq_printf(s, "group: %s\n", gname);
 			for (i = 0; i < num_pins; i++) {
 				pname = pin_get_name(pctldev, pins[i]);
-				if (WARN_ON(!pname))
+				if (WARN_ON(!pname)) {
+					mutex_unlock(&pinctrl_mutex);
 					return -EINVAL;
+				}
 				seq_printf(s, "pin %d (%s)\n", pins[i], pname);
 			}
 			seq_puts(s, "\n");