dmaengine: idxd: make dsa_bus_type const

Since commit d492cc2573a0 ("driver core: device.h: make struct
bus_type a const *"), the driver core can properly handle constant
struct bus_type, move the dsa_bus_type variable to be a constant
structure as well, placing it into read-only memory which can not be
modified at runtime.

Cc: Greg Kroah-Hartman <[e-mail]>
Suggested-by: Greg Kroah-Hartman <[e-mail]>
Signed-off-by: "Ricardo B. Marliere" <[e-mail]>
Reviewed-by: Dave Jiang <[e-mail]>
Reviewed-by: Fenghua Yu <[e-mail]>
Reviewed-by: Greg Kroah-Hartman <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Vinod Koul <[e-mail]>

drivers/dma/idxd/bus.c

···
 	return add_uevent_var(env, "MODALIAS=" IDXD_DEVICES_MODALIAS_FMT, 0);
 }
 
-struct bus_type dsa_bus_type = {
+const struct bus_type dsa_bus_type = {
 	.name = "dsa",
 	.match = idxd_config_bus_match,
 	.probe = idxd_config_bus_probe,

drivers/dma/idxd/idxd.h

···
 	iowrite32(reg.bits, idxd->reg_base + IDXD_GENCFG_OFFSET);
 }
 
-extern struct bus_type dsa_bus_type;
+extern const struct bus_type dsa_bus_type;
 
 extern bool support_enqcmd;
 extern struct ida idxd_ida;