regulator: act8945a-regulator: fix 'defined but not used' compiler warning

Fix 'defined but not used' compiler warning for act8945a_suspend()
function in case CONFIG_PM_SLEEP is not defined.

Fixes: b5ebba46 ("regulator: act8945a-regulator: add shutdown function")
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Reported-by: Andrei Stefanescu <andrei.stefanescu@microchip.com>
Signed-off-by: Mark Brown <broonie@kernel.org>