scsi: libfc: Fix variable name in fc_set_wwpn

The parameter name should be wwpn instead of wwnn.

Signed-off-by: Fam Zheng <famz@redhat.com>
Acked-by: Johannes Thumshirn <jth@kernel.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>