wlan: Rename riva to wcnss in SSR API

SSR framework renamed riva to wcnss to support pronto using the same
architecture. So make the same changes to WLAN driver.

Change-Id: I7d3d8899914e8d218786b4fadfd21c7381cb694e
CR-Fixed: 410487
diff --git a/CORE/HDD/src/wlan_hdd_main.c b/CORE/HDD/src/wlan_hdd_main.c
index 82385df..2385238 100644
--- a/CORE/HDD/src/wlan_hdd_main.c
+++ b/CORE/HDD/src/wlan_hdd_main.c
@@ -2979,7 +2979,7 @@
    if (hdd_is_ssr_required())
    {
        /* WDI timeout had happened during unload, so SSR is needed here */
-       subsystem_restart("riva");
+       subsystem_restart("wcnss");
        msleep(5000);
    }
    hdd_set_ssr_required (VOS_FALSE);