pinctrl: fix mutex deadlock in get_pinctrl_dev_from_of_node()

This obvious bug was introduced by d755910b7 ("pinctrl: move subsystem
mutex to pinctrl_dev struct").

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>