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>