scsi: qla2xxx: Add switch command to simplify fabric discovery

- add "async" gpn_ft, gnn_ft, gfpn_id, gnn_id switch commands.
- For 8G and newer adapters, use async commands when it comes to
fabric scan to reduce bottle neck.

Signed-off-by: Quinn Tran <quinn.tran@cavium.com>
Signed-off-by: Himanshu Madhani <himanshu.madhani@cavium.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>