Improve CAN FD support

Currently, the decision CAN FD or CAN 2.0 is taken by the configuration parameter CanFdKey at CAN device level. This looks incorrect to me.

The same way CAN Standard and Extended ID's can be mixed (Extended IDs have a range from 0x00000000 to 0x1FFFFFFF, with the IDE bit set), CAN 2.0 and CAN FD frames can be mixed on one bus (CAN FD has 0...64 data bytes, so CAN FD frames with e.g. 1 data bytes exists) SocketCAN's candump displays them with two-digit length:

vcan0       1AD  [01]  DC

Currently there is no way to distinguish between CAN 2.0 and CAN FD frames in QtSerialBus. I guess the following tasks are necessary:

• Extend QCanBusFrame with a flag extendedDataLength and use this to send CAN 2.0 vs. CAN FD frames, set this flag appropriate on receiving
• Change QCanBusFrame::toString() to always print
  • [x] as length for CAN 2.0 frames (0 <= x <= 8)
  • [yy] as length for CAN FD frames (00 <= yy <= 64)
• Enable CAN FD on compatible hardware to allow transferring and receiving both kinds of frames
• Another bit for "bitrate switching" has to be defined in QCanBusFrame along with the data bitrate configuration parameter