sched: move watchdog_disable() call before isolation work

Commit be45bf53
("watchdog/softlockup: Fix cpu_stop_queue_work() double-queue bug")
added wait_for_completion() call in watchdog_disable() which leads to
below issue when we try to isolate any CPU. Fix it by moving
watchdog_disable() call.

[  207.300191] BUG: sleeping function called from invalid context at \
					kernel/sched/completion.c:99
[  208.006089]  ___might_sleep+0x1c8/0x1e0
[  208.010032]  __might_sleep+0x50/0x88
[  208.013709]  wait_for_completion+0x28/0x60
[  208.017919]  watchdog_disable+0x70/0x90
[  208.021860]  do_isolation_work_cpu_stop+0x54/0x200
[  208.026784]  cpu_stopper_thread+0xac/0x150
[  208.030993]  smpboot_thread_fn+0x1c8/0x2e8
[  208.035202]  kthread+0x11c/0x130
[  208.038526]  ret_from_fork+0x10/0x1c.

Change-Id: I4d928dc03c71e68604c61c4986675fd629b69d1d
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>