[SCSI] libfcoe: add fcoe_transport structure defines to include/scsi/libfcoe.h

add the fcoe_transport struct to the common libfcoe.h header so all fcoe
transport provides can use it to attach itself as an fcoe transport. This
is the header part, and the next patch will be the transport code itself.

Signed-off-by: Yi Zou <yi.zou@intel.com>
Signed-off-by: Bhanu Prakash Gollapudi <bprakash@broadcom.com>
Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>