pinctrl: imx: Fix the logic checking if not able to find pin reg map

Current code sets "pin_reg = &info->pin_regs[i];" in each loop iteration,
so in the case of no-match, pin_reg is not NULL.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Dong Aisheng <dong.aisheng@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>