[SCSI] fcoe: fix fcoe module ref counting

Currently fcoe module ref count is used for tracking
active fcoe instances, it means each fcoe instance create
increments the count while destroy dec the count.

The dec is done only if fcoe instance is destroyed from
/sysfs but not if destroyed due to NETDEV_UNREGISTER event.
So this patch moves only module_put doing dec to common
fcoe_if_destroy function, so that dec would occur on ever
fcoe instance destroy.

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>