[SCSI] libsas: don't recover end devices attached to disabled phys

If userspace has decided to disable a phy the kernel should honor that
and not inadvertantly re-enable the phy via error recovery.  This is
more straightforward in the sata case where link recovery (via
libata-eh) is separate from sas_task cancelling in libsas-eh.  Teach
libsas to accept -ENODEV as a successful response from I_T_nexus_reset
('successful' in terms of not escalating further).

This is a more comprehensive fix then "libsas: don't recover 'gone'
devices in sas_ata_hard_reset()", as it is no longer sata-specific.

aic94xx does check the return value from sas_phy_reset() so if the phy
is disabled we proceed with clearing the I_T_nexus.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>
diff --git a/drivers/scsi/libsas/sas_init.c b/drivers/scsi/libsas/sas_init.c
index 09c14ca..120bff6 100644
--- a/drivers/scsi/libsas/sas_init.c
+++ b/drivers/scsi/libsas/sas_init.c
@@ -298,6 +298,9 @@
 	int ret;
 	enum phy_func reset_type;
 
+	if (!phy->enabled)
+		return -ENODEV;
+
 	if (hard_reset)
 		reset_type = PHY_FUNC_HARD_RESET;
 	else