fpga: dfl: use subsys_initcall to allow built-in drivers to be added

fpga: dfl: use subsys_initcall to allow built-in drivers to be added

The dfl code adds a bus. If it is built-in and there is a built-in driver
as well, the dfl module_init may be called after the driver module_init,
leading to a failure to register the driver as the bus has not been added
yet.

Use subsys_initcall, which guarantees it will be called before the drivers
init code.

Without the fix, we see failures like this:

[    0.479475] Driver 'intel-m10-bmc' was unable to register with bus_type 'dfl' because the bus was not initialized.

Cc: stable@vger.kernel.org
Fixes: 9ba3a0aa09fe ("fpga: dfl: create a dfl bus type to support DFL devices")
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@igalia.com>
Link: https://lore.kernel.org/r/20251215-dfl_subsys-v1-1-21807bad6b10@igalia.com
Reviewed-by: Xu Yilun <yilun.xu@intel.com>
Signed-off-by: Xu Yilun <yilun.xu@linux.intel.com>