heap-buffer-overflow in smc91c111_do_tx()
Description of problem:
A buffer-overflow bug was triggered by my fuzzer at smc91c111_do_tx().

I've patched hw/net/smc91c111.c with:

```
diff --git a/hw/net/smc91c111.c b/hw/net/smc91c111.c
index 702d0e8e83..286298bf06 100644
--- a/hw/net/smc91c111.c
+++ b/hw/net/smc91c111.c
@@ -429,7 +429,7 @@ static void smc91c111_writeb(void *opaque, hwaddr offset,
              /* Ignore.  */
              return;
          case 2: /* Packet Number Register */
-            s->packet_num = value;
+            s->packet_num = value & (NUM_PACKETS - 1);
              return;
          case 3: case 4: case 5:
              /* Should be readonly, but linux writes to them anyway. Ignore.  */
```

The error is:

```
==2724739==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x629000022941 at pc 0x595ebbed687b bp 0x7fffa0098a50 sp 0x7fffa0098a48
READ of size 1 at 0x629000022941 thread T0
    #0 0x595ebbed687a in smc91c111_do_tx hw/net/smc91c111.c:240:19
    #1 0x595ebbed687a in smc91c111_queue_tx hw/net/smc91c111.c:284:5
    #2 0x595ebbed687a in smc91c111_writeb hw/net/smc91c111.c:419:17
    #3 0x595ebbed687a in smc91c111_writefn hw/net/smc91c111.c:666:9
    #4 0x595ebd174d33 in memory_region_write_accessor system/memory.c:497:5
    #5 0x595ebd1744aa in access_with_adjusted_size system/memory.c:573:18
    #6 0x595ebd1738d8 in memory_region_dispatch_write system/memory.c
    #7 0x595ebd1cc984 in flatview_write_continue_step system/physmem.c:2786:18
    #8 0x595ebd1b9880 in flatview_write_continue system/physmem.c:2816:19
    #9 0x595ebd1b9880 in flatview_write system/physmem.c:2847:12
    #10 0x595ebd1b9517 in address_space_write system/physmem.c:2967:18
    #11 0x595ebc77d5c3 in qtest_process_command system/qtest.c:522:13
    #12 0x595ebc77b83b in qtest_process_inbuf system/qtest.c:776:9
    ...
```
Steps to reproduce:
```
export QEMU_ARGS="-display none -machine accel=qtest, -m 512M -machine realview-eb"
cat << EOF | ./qemu-system-arm $QEMU_ARGS -qtest /dev/null -qtest stdio
clock_step
clock_step
writel 0x4e000000 0x2b1e08f5
writew 0x4e000000 0x2b1e08f5
writel 0x4e00000c 0x66027d24
clock_step
readb 0x4e000000
writel 0x4e000008 0x238e1f29
writew 0x4e000000 0x41d9fe3b
writel 0x4e00000c 0x27022a2d
clock_step
readw 0x4e000004
clock_step
readb 0x4e000008
clock_step
writew 0x4e000000 0x620c5fdf
EOF
```
Additional information: