[MSP] Correctly report the configured and active OSD type

- Tell the OSD driver the type of displayPort, so it can be retrieved
later.
- Use OSD driver code instead of device specific code to handle
MSP_OSD_CONFIG response while minimizing driver specific code.
- Add flag for signaling the use of FrSky OSD (bit 3).
- Rename OSD_FLAGS_MAX7456_DETECTED to OSD_FLAGS_OSD_DEVICE_DETECTED.
Since we only support one OSD device type at a time, we can use the
same bit to signal wether the hardware has been detected.

src/test/unit/link_quality_unittest.cc

@@ -200,7 +200,7 @@ TEST(LQTest, TestInit)
 
     // when
     // OSD is initialised
-    osdInit(&testDisplayPort);
+    osdInit(&testDisplayPort, OSD_DISPLAYPORT_DEVICE_AUTO);
 
     // then
     // display buffer should contain splash screen