sched: Ensure watchdog is enabled before disabling

There is a race between watchdog being enabled by hotplug and core
isolation disabling the watchdog. When a CPU is hotplugged in and
the hotplug lock has been released the watchdog thread might not
have run yet to enable the watchdog.  We have to wait for the
watchdog to be enabled before proceeding.

Change-Id: I88f73603b6d389a46f8e819d9b490091d5ba4fe9
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>