dmaengine: idxd: Fix a possible NULL pointer dereference

'device_driver_attach()' dereferences its first argument (i.e. 'alt_drv')
so it must not be NULL.
Simplify the error handling logic about NULL 'alt_drv' in order to be
more robust and future-proof.

Fixes: 568b2126 ("dmaengine: idxd: fix uninit var for alt_drv")
Fixes: 6e7f3ee9 ("dmaengine: idxd: move dsa_drv support to compatible mode")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/77f0dc4f3966591d1f0cffb614a94085f8895a85.1627560174.git.christophe.jaillet@wanadoo.frSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/idxd/compat.c

@@ -45,23 +45,16 @@ static ssize_t bind_store(struct device_driver *drv, const char *buf, size_t cou
 	idxd_dev = confdev_to_idxd_dev(dev);
 	if (is_idxd_dev(idxd_dev)) {
 		alt_drv = driver_find("idxd", bus);
-		if (!alt_drv)
-			return -ENODEV;
 	} else if (is_idxd_wq_dev(idxd_dev)) {
 		struct idxd_wq *wq = confdev_to_wq(dev);
 
-		if (is_idxd_wq_kernel(wq)) {
+		if (is_idxd_wq_kernel(wq))
 			alt_drv = driver_find("dmaengine", bus);
-			if (!alt_drv)
-				return -ENODEV;
-		} else if (is_idxd_wq_user(wq)) {
+		else if (is_idxd_wq_user(wq))
 			alt_drv = driver_find("user", bus);
-			if (!alt_drv)
-				return -ENODEV;
-		} else {
-			return -ENODEV;
-		}
 	}
+	if (!alt_drv)
+		return -ENODEV;
 
 	rc = device_driver_attach(alt_drv, dev);
 	if (rc < 0)