ARM: at91: pm: simply call at91_pm_init

at91_pm_init() doesn't return a value, as is the case for its callers,
simply call it instead of returning its non-existent return value.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>