hw/net/xilinx_ethlite: Access TX_LEN register for each port

hw/net/xilinx_ethlite: Access TX_LEN register for each port

Rather than accessing the registers within the mixed RAM/MMIO
region as indexed register, declare a per-port TX_LEN. This
will help to map the RAM as RAM (keeping MMIO as MMIO) in few
commits.

Previous s->regs[R_TX_LEN0] and s->regs[R_TX_LEN1] are now
unused. Not a concern, this array will soon disappear.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@amd.com>
Message-Id: <20241112181044.92193-14-philmd@linaro.org>