scsi: bnx2fc: Fix NULL dereference in error handling

If "interface" is NULL then we can't release it and trying to will only
lead to an Oops.

Fixes: aea71a02

 ("[SCSI] bnx2fc: Introduce interface structure for each vlan interface")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>