pinctrl: pinctrl_select_state: set the old_state back on error

In unapply_new_state, the old state is re-applied, but p->state is not
set back as it should.

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>