hw/net/opencores: Clarify MMIO read/write handlers expect 32-bit access

The read/write handlers access array of 32-bit register by index:

 277 struct OpenEthState {
  ..
 287     uint32_t regs[REG_MAX];
  ..
 291 };

 546 static uint64_t open_eth_reg_read(void *opaque,
 547                                   hwaddr addr, unsigned int size)
 548 {
  ..
 551     OpenEthState *s = opaque;
 552     unsigned idx = addr / 4;
  ..
 559             v = s->regs[idx];
  ..
 563     return v;
 564 }

This is a 32-bit implementation. Make that explicit in the
MemoryRegionOps structure (this doesn't change the maximum
access size, which -- being unset -- is 64-bit).

Move the structure just after the handlers to ease code review.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20251224134644.85582-3-philmd@linaro.org>

hw/net/opencores_eth.c

@@ -683,6 +683,15 @@ static void open_eth_reg_write(void *opaque,
     }
 }
 
+static const MemoryRegionOps open_eth_reg_ops = {
+    .read = open_eth_reg_read,
+    .write = open_eth_reg_write,
+    .impl = {
+        .min_access_size = 4,
+        .max_access_size = 4,
+    },
+};
+
 static uint64_t open_eth_desc_read(void *opaque,
         hwaddr addr, unsigned int size)
 {
@@ -706,12 +715,6 @@ static void open_eth_desc_write(void *opaque,
     open_eth_check_start_xmit(s);
 }
 
-
-static const MemoryRegionOps open_eth_reg_ops = {
-    .read = open_eth_reg_read,
-    .write = open_eth_reg_write,
-};
-
 static const MemoryRegionOps open_eth_desc_ops = {
     .read = open_eth_desc_read,
     .write = open_eth_desc_write,