floppy: fix function pointer cast warnings

floppy: fix function pointer cast warnings

clang-16 complains about a control flow integrity (kcfi) violation
casting between incompatible pointers:

drivers/block/floppy.c:2001:11: error: cast from 'void (*)(void)' to 'done_f' (aka 'void (*)(int)') converts to incompatible function type [-Werror,-Wcast-function-type-strict]
 2001 |         .done           = (done_f)empty
      |                           ^~~~~~~~~~~~~

Just add another empty function with the correct prototype as a
workaround.

The warning is for code that was added before the start of the normal
git history, but I tracked it done to an early change in the reconstructed
linux-history.git.

Fixes: 598a477afe06 ("Import 1.1.41")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20240213095918.455478-1-arnd@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>