pinctrl: freescale: imx: fix bogus check of of_iomap() return value

On error path of_iomap() returns NULL, hence IS_ERR() check is invalid
and may cause a NULL pointer dereference, the change fixes this
problem.

While we are here invert a device node check to simplify the code.

Fixes: 26d8cde5 ("pinctrl: freescale: imx: add shared input select reg support")
Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Acked-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>