[SCSI] introduce sfoo_printk, sfoo_id, sfoo_channel helpers

New dev_printk wrappers, which allow us to shrink code, and
eliminate direct references to host/channel/id/lun members:
	scmd_printk()

Introduce wrappers for highly common idioms, which may also help us
eliminate some ->{channel,id} references in the future:
	{scmd,sdev}_id()
	{scmd,sdev}_channel()

The scmd_* wrappers are present in scsi/scsi_device.h because they all
employ the dereference chain cmd->device->$member.  We would prefer to
use static inline functions rather than macros, but that would have a

Rejections fixed up and
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>