dmaengine: idxd: fix uninit var for alt_drv

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: 6e7f3ee97bbe ("dmaengine: idxd: move dsa_drv support to compatible mode")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/162689250332.2114335.636367120454420852.stgit@djiang5-desk3.ch.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>