pinctrl: st: Convert to use dev_err_probe()

It's fine to call dev_err_probe() in ->probe() when error code is known.
Convert the driver to use dev_err_probe().

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>