hw/char/riscv_htif: Clarify MemoryRegionOps expect 32-bit accesses

Looking at htif_mm_ops[] read/write handlers, we notice they
expect 32-bit values to accumulate into to the 'fromhost' and
'tohost' 64-bit variables. Explicit by setting the .impl
min/max fields.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-ID: <20241129154304.34946-4-philmd@linaro.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

hw/char/riscv_htif.c

@@ -325,6 +325,10 @@ static const MemoryRegionOps htif_mm_ops = {
     .read = htif_mm_read,
     .write = htif_mm_write,
     .endianness = DEVICE_LITTLE_ENDIAN,
+    .impl = {
+        .min_access_size = 4,
+        .max_access_size = 4,
+    },
 };
 
 HTIFState *htif_mm_init(MemoryRegion *address_space, Chardev *chr,