coresight: etm4x: call cpus_read_unlock in error exit

Cpus_read_lock needs to be unlocked when setting up cpu
hotplug callback failed and exiting probe.

Change-Id: Ibbcd49d2e2d993c63b825697bc5799373f8a9a9e
Signed-off-by: Tingwei Zhang <tingwei@codeaurora.org>