watchdog: use per_cpu_ptr() in watchdog_disable()

Watchdog gets disabled from other CPUs when isolation is enabled.
Using this_cpu_ptr() would lead to undesired issues, so, switch to
per_cpu_ptr(). While at it, remove warning as it makes no sense
when core isolation is enabled.

Change-Id: Id1a59c4c88044c7570457fb55abb2dcc13de3c11
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>