dmaengine: idxd: Add wq private data accessors

Add the accessors idxd_wq_set_private() and idxd_wq_get_private()
allowing users to set and retrieve a private void * associated with an
idxd_wq.

The private data is stored in the idxd_dev.conf_dev associated with
each idxd_wq.

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Reviewed-by: Fenghua Yu <fenghua.yu@intel.com>
Acked-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>