pinctrl: mxs: Use kfree to fix build error

commit 0bf7481 (pinctrl: pinctrl-mxs: Take care of frees if the kzalloc fails)
introduced the following build error:

drivers/pinctrl/pinctrl-mxs.c:140:3: error: implicit declaration of function 'free'

Use kfree function instead.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Devendra Naga <devendra.aaru@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
diff --git a/drivers/pinctrl/pinctrl-mxs.c b/drivers/pinctrl/pinctrl-mxs.c
index afb50ee..4ba4636 100644
--- a/drivers/pinctrl/pinctrl-mxs.c
+++ b/drivers/pinctrl/pinctrl-mxs.c
@@ -137,7 +137,7 @@
 
 free_group:
 	if (!purecfg)
-		free(group);
+		kfree(group);
 free:
 	kfree(new_map);
 	return ret;