scsi: libsas: Improve vague log in SAS rediscovery

When an expander PHY which was part of a wideport disconnects, we would see
a log like this from sas_rediscover():

[   39.695554] sas: phy20 part of wide port with phy16

Here, phy20 is the PHY that disconnected, and phy16 is the lowest indexed
member PHY of the wideport.

The log implies the phy20 is still part of the wideport with phy16, so is
misleading or, at least, vague.

Improve the logs in SAS rediscovery by removing this log and adding a log
in sas_rediscover_dev() to tell what's really going on.

While we're at it, also make the logs in sas_find_bcast_dev() more
informative (and more consistent with the reset of the expander logs).

Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>