PM / wakeup: Drop redundant check from device_init_wakeup()

Since device_wakeup_disable() checks the device's power.can_wakeup
flag, device_init_wakeup() doesn't need to do that before calling it,
so drop that redundant check from device_init_wakeup().

No intentional changes in functionality.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>