dmaengine: idxd: use const struct bus_type *

dmaengine: idxd: use const struct bus_type *

In the functions unbind_store() and bind_store(), a struct bus_type *
should be a const one, as the driver core bus functions used by this
variable are expecting the pointer to be constant, and these functions
do not modify the pointer at all.

Cc: dmaengine@vger.kernel.org
Acked-by: Vinod Koul <vkoul@kernel.org>
Acked-by: Fenghua Yu <fenghua.yu@intel.com>
Acked-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/20230313182918.1312597-32-gregkh@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>