PM / wakeup: Drop redundant check from device_set_wakeup_enable()

Since both device_wakeup_enable() and device_wakeup_disable() check
if dev is not NULL and whether or not power.can_wakeup is set for it,
device_set_wakeup_enable() doesn't have to do that, so drop that
check from it.

No intentional changes in functionality.

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