ataflop: Mark expected switch fall-through

Mark switch cases where we are expecting to fall through.

This patch fixes the following warning (Building: m68k):

drivers/block/ataflop.c: In function ‘fd_locked_ioctl’:
drivers/block/ataflop.c:1728:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
   set_capacity(floppy->disk, MAX_DISK_SIZE * 2);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/block/ataflop.c:1729:2: note: here
  case FDFMTEND:
  ^~~~

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>