Explicitly send CMD_ABORT in prop mode before powering down

cpu/cc26xx-cc13xx/rf-core/prop-mode.c

@@ -968,6 +968,7 @@ off(void)
     return RF_CORE_CMD_OK;
   }
 
+  rx_off_prop();
   rf_core_power_down();
 
   /* Switch HF clock source to the RCOSC to preserve power */