arm: stellaris: make MII accesses complete immediately

When the guest attempts to start an MII register
access via the MCTL register, clear the START bit,
so that when the guest reads it back the register
transaction will be signalled as having completed.
This avoids the guest spinning as it polls the
START bit waiting for it to clear (which it
previously never would).

The  MII registers themselves still aren't implemented,
but at least we can avoid guests spending quite so much
time busy waiting.
Signed-off-by: NMichael Davidsaver <mdavidsaver@gmail.com>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Message-id: 1484938222-1423-1-git-send-email-peter.maydell@linaro.org
[PMM: expand commit message]
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

hw/net/stellaris_enet.c

@@ -416,7 +416,10 @@ static void stellaris_enet_write(void *opaque, hwaddr offset,
         s->thr = value;
         break;
     case 0x20: /* MCTL */
-        s->mctl = value;
+        /* TODO: MII registers aren't modelled.
+         * Clear START, indicating that the operation completes immediately.
+         */
+        s->mctl = value & ~1;
         break;
     case 0x24: /* MDV */
         s->mdv = value;