scsi: qedf: Fix null ptr reference in qedf_stag_change_work

Link: https://lore.kernel.org/r/20200824033436.45570-1-yebin10@huawei.com


Acked-by: Saurav Kashyap <skashyap@marvell.com>
Signed-off-by: Ye Bin <yebin10@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>