pinctrl: st: avoid multiple mutex lock

Using the sysfs inteface to inspect the pins configuration
the system can walk around a path which acquires the same
mutex twice.

On STiH407 platform, for example :
cat /sys/kernel/debug/pinctrl/920f080.pin-controller-front0/pinconf-pins
hangs the kernel and never returns.

With this patch the mutex is temporary freed.
Signed-off-by: Francesco Virlinzi <francesco.virlinzi@st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/pinctrl/pinctrl-st.c

@@ -1012,8 +1012,10 @@ static void st_pinconf_dbg_show(struct pinctrl_dev *pctldev,
 				   struct seq_file *s, unsigned pin_id)
 {
 	unsigned long config;
-	st_pinconf_get(pctldev, pin_id, &config);
 
+	mutex_unlock(&pctldev->mutex);
+	st_pinconf_get(pctldev, pin_id, &config);
+	mutex_lock(&pctldev->mutex);
 	seq_printf(s, "[OE:%ld,PU:%ld,OD:%ld]\n"
 		"\t\t[retime:%ld,invclk:%ld,clknotdat:%ld,"
 		"de:%ld,rt-clk:%ld,rt-delay:%ld]",