PM: wakeup: Unify device_init_wakeup() for PM_SLEEP and !PM_SLEEP

Previously the CONFIG_PM_SLEEP and !CONFIG_PM_SLEEP device_init_wakeup()
implementations differed in confusing ways:

  - The PM_SLEEP version checked for a NULL device pointer and returned
    -EINVAL, while the !PM_SLEEP version did not and would simply
    dereference a NULL pointer.

  - When called with "false", the !PM_SLEEP version cleared "capable" and
    "enable" in the opposite order of the PM_SLEEP version.  That was
    harmless because for !PM_SLEEP they're simple assignments, but it's
    unnecessary confusion.

Use a simplified version of the PM_SLEEP implementation for both cases.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>