[SCSI] fcoe: initialize return value in fcoe_destroy

When doing echo ethX > /sys..../destroy I am getting
errors when the tear down succeeds. It looks like the
reason for this is because the rc var is not getting set
when the destruction works. This just sets it to zero.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>