powerpc/papr_scm: Mark papr_scm_ndctl() as static

Function papr_scm_ndctl() is neither exported from the module nor
called directly from outside 'papr.c' hence should be marked 'static'.

Signed-off-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200130040206.79998-1-vaibhav@linux.ibm.com