scsi: libsas: no need to join wide port again in sas_ex_discover_dev()

Since we are processing events synchronously now, the second call of
sas_ex_join_wide_port() in sas_ex_discover_dev() is not needed. There will
be no races with other works in disco workqueue. So remove the second
sas_ex_join_wide_port().

I did not change the return value of 'res' to error when discover failed
because we need to continue to discover other phys if one phy discover
failed. So let's keep that logic as before and just add a debug log to
detect the failure. And directly return if second fanout expander attatched
to the parent expander because it has nothing to do after the phy is
disabled.

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