[SCSI] libsas: do not set res = 0 in sas_ex_discover_dev()

We should not set res to 0 in function sas_ex_discover_dev  in order to let
it discover it further when wide port hotplug in .

Signed-off-by: Tom Peng <tom_peng@usish.com>
Signed-off-by: Jack Wang <jack_wang@usish.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>