powerpc/papr_scm: Fix build error due to wrong printf specifier

When I changed the rc variable to be long rather than int64_t I
neglected to update the printk(), leading to a build break:

  arch/powerpc/platforms/pseries/papr_scm.c: In function 'papr_scm_pmem_flush':
  arch/powerpc/platforms/pseries/papr_scm.c:144:26: warning: format
    '%lld' expects argument of type 'long long int', but argument 3 has
    type 'long int' [-Wformat=]

Fixes: 75b7c05e ("powerpc/papr_scm: Implement support for H_SCM_FLUSH hcall")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210416111209.765444-2-mpe@ellerman.id.au