hw/net/can: fix Xilinx ZynqMP CAN RX FIFO logic

hw/net/can: fix Xilinx ZynqMP CAN RX FIFO logic

For consistency, function "update_rx_fifo()" should use the RX FIFO
register field names, not the TX FIFO ones, even if they refer to the
same bit positions in the register.

Signed-off-by: Anton Kochkov <anton.kochkov@proton.me>
Reviewed-by: Francisco Iglesias <frasse.iglesias@gmail.com>
Message-id: 20220817141754.2105981-1-anton.kochkov@proton.me
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1123
[PMM: tweaked commit message]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>