pinctrl: imx: remove an unnecessary NULL check

The address of "ipctl->pin_regs[pin_id]" can't be NULL.  It's the offset
into an array in the middle of a struct.  This patch removes the check.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>