dmaengine: idxd: fix off by one on cdev dwq refcount

dmaengine: idxd: fix off by one on cdev dwq refcount

The refcount check for dedicated workqueue (dwq) is off by one and allows
more than 1 user to open the char device. Fix check so only a single user
can open the device.

Fixes: 42d279f9137a ("dmaengine: idxd: add char driver to expose submission portal to userland")
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/158403020187.10208.14117394394540710774.stgit@djiang5-desk3.ch.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>