blk-mq: use switch/case to improve readability in blk_mq_try_issue_list_directly

Use switch/case handle error as other function do to improve
readability in blk_mq_try_issue_list_directly.

Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>