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: e2729113

 ("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>