cpu-hotplug: Fix the cpus_write_lock usage

'Commit ("5f7b7a9f cpu-hotplug:
Keep atleast 1 online and un-isolated CPU")'
caused regression in CPU hotplug scenario. cpus_write_lock
taken and not released in error path. So fix
the sequence of checks in _cpu_down function.

Change-Id: I96cb3c2f1de93992af0e4f1d85857744b65e7a21
Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.org>