powerpc/papr_scm: Ensure rc is always initialized in papr_scm_pmu_register()

Clang warns:

  arch/powerpc/platforms/pseries/papr_scm.c:492:6: warning: variable 'rc' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
          if (!p->stat_buffer_len)
              ^~~~~~~~~~~~~~~~~~~
  arch/powerpc/platforms/pseries/papr_scm.c:523:64: note: uninitialized use occurs here
          dev_info(&p->pdev->dev, "nvdimm pmu didn't register rc=%d\n", rc);
                                                                        ^~
  include/linux/dev_printk.h:150:67: note: expanded from macro 'dev_info'
          dev_printk_index_wrap(_dev_info, KERN_INFO, dev, dev_fmt(fmt), ##__VA_ARGS__)
                                                                          ^~~~~~~~~~~
  include/linux/dev_printk.h:110:23: note: expanded from macro 'dev_printk_index_wrap'
                  _p_func(dev, fmt, ##__VA_ARGS__);                       \
                                      ^~~~~~~~~~~
  arch/powerpc/platforms/pseries/papr_scm.c:492:2: note: remove the 'if' if its condition is always false
          if (!p->stat_buffer_len)
          ^~~~~~~~~~~~~~~~~~~~~~~~
  arch/powerpc/platforms/pseries/papr_scm.c:484:8: note: initialize the variable 'rc' to silence this warning
          int rc, nodeid;
                ^
                = 0
  1 warning generated.

The call to papr_scm_pmu_check_events() was eliminated but a return code
was not added to the if statement. Add the same return code from
papr_scm_pmu_check_events() for this condition so there is no more
warning.

Fixes: 9b1ac046 ("powerpc/papr_scm: Fix nvdimm event mappings")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://github.com/ClangBuiltLinux/linux/issues/1701
Link: https://lore.kernel.org/r/20220830151256.1473169-1-nathan@kernel.org