scsi: fnic: Remove set but not used 'old_vlan'

scsi: fnic: Remove set but not used 'old_vlan'

This addresses the following gcc warning with "make W=1":

drivers/scsi/fnic/fnic_main.c: In function ‘fnic_set_vlan’:
drivers/scsi/fnic/fnic_main.c:555:6: warning: variable ‘old_vlan’ set
but not used [-Wunused-but-set-variable]
  555 |  u16 old_vlan;
      |      ^~~~~~~~

Link: https://lore.kernel.org/r/20200831081126.3251288-2-yanaijie@huawei.com
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>