pm: Avoid unnecessary check in SMP

_current_cpu is a macro that expands to an assert that checks if the
current context can be migrated to a another cpu. Since this
pm_system_suspend() is called from the idle thread (each cpu has its
own) and  with locked this check is redundant. Just use
arch_curr_cpu() to avoid it.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>