AP_CANManager: set canfd bitrates using CANFD_SUPPORTED opt

also adds option to set as 4MBits/s

libraries/AP_CANManager/AP_CANIfaceParams.cpp

@@ -38,10 +38,10 @@ const AP_Param::GroupInfo AP_CANManager::CANIface_Params::var_info[] = {
 #if HAL_CANFD_SUPPORTED
     // @Param: BITRATE
     // @DisplayName: Bitrate of CAN interface
-    // @Description: Bit rate can be set up to from 10000 to 8000000
-    // @Values: 2:2M, 5:5M, 8:8M
+    // @Description: Bit rate can be set up to from 1000000 to 8000000
+    // @Values: 1:1M, 2:2M, 4:4M, 5:5M, 8:8M
     // @User: Advanced
-    AP_GROUPINFO("FDBITRATE", 3, AP_CANManager::CANIface_Params, _fdbitrate, 8),
+    AP_GROUPINFO("FDBITRATE", 3, AP_CANManager::CANIface_Params, _fdbitrate, HAL_CANFD_SUPPORTED),
 #endif
     // Index 3 occupied by Param: DEBUG
     AP_GROUPEND