eth: size EMAC RX DMA pool above the TCP window (1600B x 24)

WND=23040 alone regressed: sender fps 20 -> 14, drop-oldest 3x,
recv bimodal (min 20ms but 200-450ms stalls; firmware wire_min
6.2Mbps / wire_max 75Mbps, recv_max 253ms). The stalls are RTO
recovery: the default EMAC RX pool (20 x 512B = 10KB) is smaller
than the 23KB the window now invites in flight, so a full-window
burst overruns the RX descriptors, the burst tail is dropped with
no dup-ACKs behind it, and the sender waits out a ~200ms min-RTO.

1600B buffers fit one MSS frame per buffer (1 descriptor/packet
instead of 3); 24 of them = 38.4KB >= window + slack. ~44KB more
internal RAM (491KB free).

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