hw/net/can/xlnx-versal-canfd: Handle flags correctly

hw/net/can/xlnx-versal-canfd: Handle flags correctly

Add support for QEMU_CAN_FRMF_ESI and QEMU_CAN_FRMF_BRS flags, and
ensure frame->flags is always initialized to 0.

Note that the Xilinx IP core doesn't allow manually setting the ESI bit
during transmits, so it's only implemented for the receive case.

Signed-off-by: Doug Brown <doug@schmorgal.com>
Reviewed-by: Pavel Pisa <pisa@cmp.felk.cvut.cz>
Message-id: 20240827034927.66659-5-doug@schmorgal.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>