coresight: remove hotplug locking from etm probe

Calling register_hotcpu_notifier inside of get/put_online_cpus
i.e. after taking the cpu_hotplug lock results in a deadlock
since register_hotcpu_notifier takes the cpu_add_remove_lock
which has already been acquired by the hotplug code before
requesting the cpu_hotplug lock.

Remove the get/put_online_cpus calls since they are not required.

CRs-Fixed: 501374
Change-Id: I3fb58265f6fedeb696d035d281af0d7904e28b79
Signed-off-by: Pratik Patel <pratikp@codeaurora.org>