scsi: csiostor: Remove set but not used variable 'pln'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/scsi/csiostor/csio_attr.c: In function 'csio_fcoe_free_vnp':
drivers/scsi/csiostor/csio_attr.c:500:21: warning:
 variable 'pln' set but not used [-Wunused-but-set-variable]

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Varun Prakash <varun@chelsio.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>