pinctrl: pinctrl-mxs: set platform driver data to NULL at errpath and at unregister

clear the platform data pointer when mxs_pinctrl_probe_dt fails,
and also before the unregistering with pinctrl subsystem.

Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>