platform/x86: intel_pmc_core: Fix file permissions for ltr_show

File permissions for ltr_show attribute should be similar to other
debugfs attributes created by this driver. '0644' should be used only
when there is a write operation desired such as for ltr_ignore.

Cc: "David E. Box" <david.e.box@intel.com>
Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Fixes: 2eb15055 ("platform/x86: intel_pmc_core: Show Latency Tolerance info")
Signed-off-by: Rajneesh Bhardwaj <rajneesh.bhardwaj@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>