nvmet-fc: fix kernel-doc warning for nvmet_fc_unregister_targetport

This fixes following kernel-doc warning:-

drivers/nvme/target/fc.c:1619: warning: expecting prototype for nvme_fc_unregister_targetport(). Prototype was for nvmet_fc_unregister_targetport() instead

Signed-off-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>