UPSTREAM: PM / wakeup: Drop wakeup_source_init(), wakeup_source_prepare()

wakeup_source_init() has no users. Remove it.

As a result, wakeup_source_prepare() is only called from
wakeup_source_create(). Merge wakeup_source_prepare() into
wakeup_source_create() and remove it.

Change wakeup_source_create() behavior so that assigning NULL to wakeup
source's name throws an error.

Signed-off-by: Tri Vo <trong@android.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
(cherry picked from commit 0d105d0f25386ee5b74603db6d249ebed7590cbc)
Bug: 129087298
Signed-off-by: Tri Vo <trong@google.com>
Change-Id: I939f8e2c40951720d0f33eda2be671f8ce06101c