dmaengine: idxd: set descriptor allocation size to threshold for swq

Since submission is sent to limited portal, the actual wq size for shared
wq is set by the threshold rather than the wq size. When the wq type is
shared, set the allocated descriptors to the threshold.

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/162827151733.3459223.3829837172226042408.stgit@djiang5-desk3.ch.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>