x86: pmc-atom: Assign debugfs node as soon as possible

pmc_dbgfs_unregister() will be called when pmc->dbgfs_dir is unconditionally
NULL on error path in pmc_dbgfs_register(). To prevent this we move the
assignment to where is should be.

Fixes: f855911c1f48 (x86/pmc_atom: Expose PMC device state and platform sleep state)
Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Aubrey Li <aubrey.li@linux.intel.com>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: Kumar P. Mahesh <mahesh.kumar.p@intel.com>
Link: http://lkml.kernel.org/r/1421253575-22509-2-git-send-email-andriy.shevchenko@linux.intel.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>