dmaengine: idxd: make idxd_wq_enable() return 0 if wq is already enabled

When calling idxd_wq_enable() and wq is already enabled, code should return 0
and indicate function is successful instead of return error code and fail.
This should also put idxd_wq_enable() in sync with idxd_wq_disable() where
it returns 0 if wq is already disabled.

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/165090980906.1378449.1939401700832432886.stgit@djiang5-desk3.ch.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>