cxl/core: Fix cxl_device_lock() class detection

If cxl_device_lock() is used on a non-CXL device the expectation is that
the lock class will fall back to CXL_ANON_LOCK. Instead it crashes when
trying to determine if the device is a 'decoder'. Specifically when the
device has a NULL type pointer. Just check for NULL before
de-referencing ->release.

Fixes: 3c5b903955 ("cxl: Prove CXL locking")
Reported-by: Ben Widawsky <ben.widawsky@intel.com>
Reviewed-by: Ben Widawsky <ben.widawsky@intel.com>
Link: https://lore.kernel.org/r/164439225406.2941117.3927102269866914339.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/cxl/core/port.c

@@ -284,7 +284,7 @@ EXPORT_SYMBOL_NS_GPL(is_root_decoder, CXL);
 
 bool is_cxl_decoder(struct device *dev)
 {
-	return dev->type->release == cxl_decoder_release;
+	return dev->type && dev->type->release == cxl_decoder_release;
 }
 EXPORT_SYMBOL_NS_GPL(is_cxl_decoder, CXL);