pinctrl: pinctrl-imx: don't use invalid value of conf_reg

The right check for conf_reg to be invalid it testing against -1 not 0
as is done in the rest of the driver.

This fixes an oops that can be triggered by:

	cat /sys/kernel/debug/pinctrl/43fac000.iomuxc/*

Fixes: ae75ff814538 ("pinctrl: pinctrl-imx: add imx pinctrl core driver")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>