pinctrl: imx: remove duplicated const

fix smatch warning:
drivers/pinctrl/pinctrl-imx.c:435:21: warning: duplicate const

Signed-off-by: Richard Zhao <richard.zhao@freescale.com>
Acked-by: Dong Aisheng <dong.aisheng@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>