scsi: libsas: make the event threshold configurable

Add a sysfs attr that LLDD can configure it for every host. We made an
example in hisi_sas. Other LLDDs using libsas can implement it if they
want.

Suggested-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
CC: John Garry <john.garry@huawei.com>
CC: Johannes Thumshirn <jthumshirn@suse.de>
CC: Ewan Milne <emilne@redhat.com>
CC: Christoph Hellwig <hch@lst.de>
CC: Tomas Henzl <thenzl@redhat.com>
CC: Dan Williams <dan.j.williams@intel.com>
Acked-by: John Garry <john.garry@huawei.com> #for hisi_sas part
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

include/scsi/libsas.h

@@ -681,6 +681,7 @@ extern int sas_bios_param(struct scsi_device *,
 			  sector_t capacity, int *hsc);
 extern struct scsi_transport_template *
 sas_domain_attach_transport(struct sas_domain_function_template *);
+extern struct device_attribute dev_attr_phy_event_threshold;
 
 int  sas_discover_root_expander(struct domain_device *);