PM-wakeup: Delete unnecessary checks before three function calls

The following functions test whether their argument is NULL
and then return immediately.
 * dev_pm_arm_wake_irq
 * dev_pm_disarm_wake_irq
 * wakeup_source_unregister

Thus the test around the calls is not needed.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Acked-by: Pavel Machek <pavel@ucw.cz>
[ rjw: Minor whitespace adjustments ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>