dmaengine: idxd: fix uninit var for alt_drv

0-day detected uninitialized alt_drv variable in the bind_store() function.
The branch can be taken when device is not idxd device or wq 'struct
device'. Init alt_drv to NULL.

Fixes: 6e7f3ee9 ("dmaengine: idxd: move dsa_drv support to compatible mode")
Reported-by: Nkernel test robot <lkp@intel.com>
Signed-off-by: NDave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/162689250332.2114335.636367120454420852.stgit@djiang5-desk3.ch.intel.comSigned-off-by: NVinod Koul <vkoul@kernel.org>

drivers/dma/idxd/compat.c

@@ -34,7 +34,7 @@ static ssize_t bind_store(struct device_driver *drv, const char *buf, size_t cou
 {
 	struct bus_type *bus = drv->bus;
 	struct device *dev;
-	struct device_driver *alt_drv;
+	struct device_driver *alt_drv = NULL;
 	int rc = -ENODEV;
 	struct idxd_dev *idxd_dev;