floppy: Fix fall-through warning for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a break statement instead of letting the code fall
through to the next case.

Link: https://github.com/KSPP/linux/issues/115
Link: https://lore.kernel.org/linux-hardening/47bcd36a-6524-348b-e802-0691d1b3c429@kernel.dk/


Suggested-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Denis Efremov <efremov@linux.com>