drivers: base: power: remove wakeup_sources_stats_dentry variable

wakeup_sources_stats_dentry is assigned when the debugfs file is
created, but then never used ever again.  So no need for it at all, just
remove it and call debugfs_create_file() on its own.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>