hw/net/can/xlnx-zynqmp: Avoid underflow while popping RX FIFO

hw/net/can/xlnx-zynqmp: Avoid underflow while popping RX FIFO

Per https://docs.xilinx.com/r/en-US/ug1085-zynq-ultrascale-trm/Message-Format

  Message Format

  The same message format is used for RXFIFO, TXFIFO, and TXHPB.
  Each message includes four words (16 bytes). Software must read
  and write all four words regardless of the actual number of data
  bytes and valid fields in the message.

There is no mention in this reference manual about what the
hardware does when not all four words are read. To fix the
reported underflow behavior, I choose to fill the 4 frame data
registers when the first register (ID) is accessed, which is how
I expect hardware would do.

Reported-by: Qiang Liu <cyruscyliu@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Francisco Iglesias <francisco.iglesias@amd.com>
Reviewed-by: Vikram Garhwal <vikram.garhwal@amd.com>
Message-id: 20231124183325.95392-3-philmd@linaro.org
Fixes: 98e5d7a2b7 ("hw/net/can: Introduce Xilinx ZynqMP CAN controller")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1427
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Francisco Iglesias <francisco.iglesias@amd.com>
Reviewed-by: Vikram Garhwal <vikram.garhwal@amd.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>