[SCSI] fcoe: cleans up fcoe_disable and fcoe_enable

The fc_fabric_logoff and fc_fabric_login are redundant
here after recently added fcoe_ctlr_link_down/up to
these functions, therefore this patch removes logoff
and login to only use link down and up here. This works
best for their current usages with fcoe DCB link down or up.

This also works well to avoid EIO errors when fcoe DCB link
goes down as lport state moves out of ready quickly from
fcoe_ctlr_link_down and that allows re-queuing timed out IOs
for this case also.

Signed-off-by: Vasu Dev <vasu.dev@intel.com>
Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>