[SCSI] bsg: separate bsg and SCSI (so SCSI can be modular)

This patch moves the bsg registration into SCSI so that bsg no longer
has a dependency on the scsi_interface_register API.

This can be viewed as a temporary expedient until we can get universal
bsg binding sorted out properly.  Also use the sdev bus_id as the
generic bsg name (to avoid clashes with the queue name).

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