scsi: lpfc: Fix NULL pointer dereference during PCI error recovery

Recent commit on patchset "lpfc updates for 11.2.0.14" fixed an issue
about dereferencing a NULL pointer on port reset. The specific commit,
named "lpfc: Fix system crash when port is reset.", is missing a check
against NULL pointer on lpfc_els_flush_cmd() though.

Since we destroy the queues on adapter resets, like in PCI error
recovery path, we need the validation present on this patch in order to
avoid a NULL pointer dereference when trying to flush commands of ELS
wq, after it has been destroyed (which would lead to a kernel oops).

Tested-by: Raphael Silva <raphasil@linux.vnet.ibm.com>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@linux.vnet.ibm.com>
Acked-by: James Smart <james.smart@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>