powerpc/perf: Change the data type for the variable 'ncpu' in IMC code

Change the data type for the variable 'ncpu' in ppc_core_imc_cpu_offline(),
since cpumask_any_but() returns an 'int' value.

Signed-off-by: Anju T Sudhakar <anju@linux.vnet.ibm.com>
Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>