PM: sleep: wakeup: Skip wakeup_source_sysfs_remove() if device is not there

Skip wakeup_source_sysfs_remove() to fix a NULL pinter dereference via
ws->dev, if the wakeup source is unregistered before registering the
wakeup class from device_add().

Fixes: 2ca3d1ec ("PM / wakeup: Register wakeup class kobj after device is added")
Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: 5.4+ <stable@vger.kernel.org> # 5.4+
[ rjw: Subject & changelog, white space ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>