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: e2729113ce66 ("platform/x86: Intel PMT class driver")
Signed-off-by: Dan Carpenter <[e-mail]>
Reviewed-by: David E. Box <[e-mail]>
Link: https://lore.kernel.org/r/20201117072251.GC1111239@mwanda
Signed-off-by: Hans de Goede <[e-mail]>

drivers/platform/x86/intel_pmt_class.c

···
 		return 0;
 
 fail_ioremap:
-	sysfs_remove_group(entry->kobj, ns->attr_grp);
+	if (ns->attr_grp)
+		sysfs_remove_group(entry->kobj, ns->attr_grp);
 fail_sysfs:
 	device_unregister(dev);
 fail_dev_create: