[SCSI] fcoe: Remove reference counting on 'stuct fcoe_interface'

The reference counting was necessary on these instances
because it was possible for NPIV ports to be destroyed
after the N_Port. A previous patch ensures that all NPIV
ports are destroyed before the N_Port making the need to
track references on the interface unnecessary.

Signed-off-by: Robert Love <robert.w.love@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>