platform/x86: pmt: Fix a potential Oops on error in probe

The "ns->attr_grp" pointer can be NULL so this error handling code needs
to check for that to avoid an Oops.

Fixes: e2729113ce ("platform/x86: Intel PMT class driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: David E. Box <david.e.box@linux.intel.com>
Link: https://lore.kernel.org/r/20201117072251.GC1111239@mwanda
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

drivers/platform/x86/intel_pmt_class.c

@@ -225,6 +225,7 @@ static int intel_pmt_dev_register(struct intel_pmt_entry *entry,
 		return 0;
 
 fail_ioremap:
+	if (ns->attr_grp)
 		sysfs_remove_group(entry->kobj, ns->attr_grp);
 fail_sysfs:
 	device_unregister(dev);