ataflop: fix error handling in atari_floppy_init()

Smatch complains that there is an off by one if the allocation fails in:

	DMABuffer = atari_stram_alloc(BUFFER_SIZE+512, "ataflop");

In that situation, "i" would be point to one element beyond the end of
the unit[] array.

There is a second bug because the error handling calls
blk_mq_free_tag_set(&unit[i].tag_set); regardless of whether
"disk->queue" is NULL or non-NULL.  So if blk_mq_init_sq_queue() fails,
then that means unit[i].tag_set->tags is NULL and it leads to an Oops.

It's easiest to call put_disk() before the goto to clean up the partial
iteration.  Then the earlier unit[] elements are fully allocated so we
can remove the checks whether "disk->queue" is NULL and the code is
simpler.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>