docs: driver-api: firmware_loader: fix missing argument in usage example

By the time firmware-upload support landed in commit 97730bbb
("firmware_loader: Add firmware-upload support"), the arguments for
firmware_upload_register() had changed, and while this is automagically
represented in the kernel doc bits, the usage example was not kept in
sync.
Add the missing argument as per the driver.

Fixes: 97730bbb ("firmware_loader: Add firmware-upload support")
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230329123425.4177084-1-conor.dooley@microchip.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>