fpga: dfl: make dfl_bus_type const

fpga: dfl: make dfl_bus_type const

Now that the driver core can properly handle constant struct bus_type,
move the dfl_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 <[email protected]>
Suggested-by: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Ricardo B. Marliere <[email protected]>
Reviewed-by: Greg Kroah-Hartman <[email protected]>
Acked-by: Xu Yilun <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Xu Yilun <[email protected]>