BLE: Replace Serial with RawSerial in Cordio H4 Transport Driver

Author: Donatien Garnier

features/FEATURE_BLE/targets/TARGET_CORDIO/driver/H4TransportDriver.h

@@ -69,7 +69,7 @@ class H4TransportDriver : public CordioHCITransportDriver {
 private:
     void on_controller_irq();
 
-    Serial uart;
+    RawSerial uart;
     PinName cts;
     PinName rts;
 };