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>

drivers/block/floppy.c

@@ -530,14 +530,13 @@ static struct format_descr format_req;
 static char *floppy_track_buffer;
 static int max_buffer_sectors;
 
-typedef void (*done_f)(int);
 static const struct cont_t {
 	void (*interrupt)(void);
 				/* this is called after the interrupt of the
 				 * main command */
 	void (*redo)(void);	/* this is called to retry the operation */
 	void (*error)(void);	/* this is called to tally an error */
-	done_f done;		/* this is called to say if the operation has
+	void (*done)(int);	/* this is called to say if the operation has
 				 * succeeded/failed */
 } *cont;
 
@@ -985,6 +984,10 @@ static void empty(void)
 {
 }
 
+static void empty_done(int result)
+{
+}
+
 static void (*floppy_work_fn)(void);
 
 static void floppy_work_workfn(struct work_struct *work)
@@ -1998,14 +2001,14 @@ static const struct cont_t wakeup_cont = {
 	.interrupt	= empty,
 	.redo		= do_wakeup,
 	.error		= empty,
-	.done		= (done_f)empty
+	.done		= empty_done,
 };
 
 static const struct cont_t intr_cont = {
 	.interrupt	= empty,
 	.redo		= process_fd_request,
 	.error		= empty,
-	.done		= (done_f)empty
+	.done		= empty_done,
 };
 
 /* schedules handler, waiting for completion. May be interrupted, will then