scsi: libsas: Do discovery on empty PHY to update PHY info

When we discover the PHY is empty in sas_rediscover_dev(), the PHY
information (like negotiated linkrate) is not updated.

As such, for a user examining sysfs for that PHY, they would see
incorrect values:

root@(none)$ cd /sys/class/sas_phy/phy-0:0:20
root@(none)$ more negotiated_linkrate
3.0 Gbit
root@(none)$ echo 0 > enable
root@(none)$ more negotiated_linkrate
3.0 Gbit

So fix this, simply discover the PHY again, even though we know it's empty;
in the above example, this gives us:

root@(none)$ more negotiated_linkrate
Phy disabled

We must do this after unregistering the device associated with the PHY
(in sas_unregister_devs_sas_addr()).

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