hw/net/lan9118: Replace magic '2048' value by MIL_TXFIFO_SIZE definition

hw/net/lan9118: Replace magic '2048' value by MIL_TXFIFO_SIZE definition

The magic 2048 is explained in the LAN9211 datasheet (DS00002414A)
in chapter 1.4, "10/100 Ethernet MAC":

  The MAC Interface Layer (MIL), within the MAC, contains a
  2K Byte transmit and a 128 Byte receive FIFO which is separate
  from the TX and RX FIFOs. [...]

Note, the use of the constant in lan9118_receive() reveals that
our implementation is using the same buffer for both tx and rx.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20240409133801.23503-2-philmd@linaro.org>
(cherry picked from commit a45223467e4e185fff1c76a6483784fa379ded77)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>