floppy: cleanups: use memset() to zero reply_buffer

Use memset() to zero reply buffer in raw_cmd_copyin() instead
of a for loop.

Signed-off-by: Denis Efremov <efremov@linux.com>
Link: https://lore.kernel.org/r/20210416083449.72700-4-efremov@linux.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>