floppy: remove dr, reuse drive on do_floppy_init

This is a small cleanup, that also may turn error handling of
unitialized disks more readable. We don't need a separate variable to
track allocated disks, remove dr and reuse drive variable instead.

Signed-off-by: Herton Ronaldo Krzesinski <herton.krzesinski@canonical.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>