bsg: device hash table cleanup

- kill unused bsg_list_idx macro.
- add bsg_dev_idx_hash() that returns an appropriate hlist_head.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>