xen/blkback: Fix warning error.

drivers/block/xen-blkback/xenbus.c: In function 'xen_blkbk_discard':
drivers/block/xen-blkback/xenbus.c:419:4: warning: passing argument 1 of 'dev_warn' makes pointer from integer without a cast
+[enabled by default]
include/linux/device.h:894:5: note: expected 'const struct device *' but argument is of type 'long int'

It is unclear how that mistake made it in. It surely is wrong.
Acked-by: NJens Axboe <axboe@kernel.dk>
Reported-by: NStephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/block/xen-blkback/xenbus.c

@@ -416,7 +416,7 @@ static void xen_blkbk_discard(struct xenbus_transaction xbt, struct backend_info
 				    "discard-secure", "%d",
 				    blkif->vbd.discard_secure);
 		if (err) {
-			dev_warn(dev-dev, "writing discard-secure (%d)", err);
+			dev_warn(&dev->dev, "writing discard-secure (%d)", err);
 			return;
 		}
 	}