Correct open error for ts850 backend

Like the ts570 backend, the ts850 backend did not define a function for
rig_open in its rig_caps.  This is now fixed.  Confirmed by Ervin,
HA2OS.

kenwood/ts850.c

@@ -208,6 +208,7 @@ const struct rig_caps ts850_caps = {
 	.priv =  (void *)&ts850_priv_caps,
 
         .rig_init = kenwood_init,
+        .rig_open = kenwood_open,
         .rig_cleanup = kenwood_cleanup,
 	.set_freq =  kenwood_set_freq,
 	.get_freq =  kenwood_get_freq,