pinctrl: pinctrl-single: fix compile warning when no CONFIG_PM

This warning has been introduced by the commit
 0f9bc4bcdf4f pinctrl: single: adopt pinctrl sleep mode management

Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>