floppy: cleanups: use ST0 as reply_buffer index 0

Use ST0 as 0 index for reply_buffer array. get_fdc_version() is the only
function that uses index 0 directly instead of the ST0 define.

Signed-off-by: Denis Efremov <efremov@linux.com>
Link: https://lore.kernel.org/r/20210416083449.72700-3-efremov@linux.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>