scsi: libcxgbi: return error if interface is not up

Do not post hw active open cmd if IFF_UP is not set or link is down on
the interface, return -ENETDOWN in this case.

Signed-off-by: Varun Prakash <varun@chelsio.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>