sched: Don't fail isolation request for an already isolated CPU

When isolating a CPU, a check is performed to see if there is
only 1 active CPU in the system. If that is the case, the
CPU is not isolated. However this check is done before testing
if the requested CPU is already isolated or not. If the
requested CPU is already isolated, there is no need to fail
the isolation even when there is only 1 active CPU in the system.

For example, 0-6 CPUs are isolated on a 8 CPU machine. When
an isolation request comes for CPU6, which is already isolated,
the current code fail the requesting thinking we end up
with no active CPU in the system.

Change-Id: I28fea4ff67ffed82465e5cfa785414069e4a180a
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>