scsi: lpfc: Avoid NULL pointer dereference in lpfc_els_abort()

We might have a NULL pring in lpfc_els_abort(), for example on error
recovery path, since queues are destroyed during error recovery
mechanism.

In this case, we should just drop the abort since the queues will be
recreated anyway. This patch just verifies for NULL pointer and stop the
abortion of the queue in case of a NULL pring.

Also, this patch converts return type of lpfc_els_abort() from int to
void, since it's not checked anywhere.

Reported-by: Harsha Thyagaraja <hathyaga@in.ibm.com>
Reported-by: Naresh Bannoth <nbannoth@in.ibm.com>
Tested-by: Raphael Silva <raphasil@linux.vnet.ibm.com>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@linux.vnet.ibm.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Acked-by: James Smart <james.smart@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>