hw/net/can/xlnx-versal-canfd: Fix FIFO issues

hw/net/can/xlnx-versal-canfd: Fix FIFO issues

The read index should not be changed when storing a new message into the
RX or TX FIFO. Changing it at this point will cause the reader to get
out of sync. The wrapping of the read index is already handled by the
pre-write functions for the FIFO status registers anyway.

Additionally, the calculation for wrapping the store index was off by
one, which caused new messages to be written to the wrong location in
the FIFO. This caused incorrect messages to be delivered.

Signed-off-by: Doug Brown <doug@schmorgal.com>
Reviewed-by: Francisco Iglesias <francisco.iglesias@amd.com>
Message-id: 20240827034927.66659-8-doug@schmorgal.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>