dmaengine: idxd: add RO check for wq max_transfer_size write

Block wq_max_transfer_size_store() when the device is configured as
read-only and not configurable.

Fixes: d7aad555 ("dmaengine: idxd: add support for configurable max wq xfer size")
Reported-by: NBernice Zhang <bernice.zhang@intel.com>
Tested-by: NBernice Zhang <bernice.zhang@intel.com>
Signed-off-by: NDave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/164971488154.2200913.10706665404118545941.stgit@djiang5-desk3.ch.intel.comSigned-off-by: NVinod Koul <vkoul@kernel.org>

drivers/dma/idxd/sysfs.c

@@ -905,6 +905,9 @@ static ssize_t wq_max_transfer_size_store(struct device *dev, struct device_attr
 	u64 xfer_size;
 	int rc;
 
+	if (!test_bit(IDXD_FLAG_CONFIGURABLE, &idxd->flags))
+		return -EPERM;
+
 	if (wq->state != IDXD_WQ_DISABLED)
 		return -EPERM;