ARM: shmobile: rcar-gen2: correct return value of shmobile_smp_apmu_suspend_init

The dummy shmobile_smp_apmu_suspend_init() function provided when
CPU_IDLE is not set should not return a value as per the signature
of the function.

This problem appears to have been introduced by
867ba81f728f1daa ("ARM: shmobile: APMU: Add Core-Standby-state for Suspend
to RAM").

Cc: Keita Kobayashi <keita.kobayashi.ym@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>