hw/arm/nseries: convert ffs(3) to ctz32()

It is not clear from the code how a 0 parameter should be handled by the
hardware.  Keep the same behavior as ffs(0) - 1 == -1.

Cc: Andrzej Zaborowski <balrog@zabor.org>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Message-id: 1427124571-28598-3-git-send-email-stefanha@redhat.com
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

hw/arm/nseries.c

@@ -579,7 +579,10 @@ static uint32_t mipid_txrx(void *opaque, uint32_t cmd, int len)
 
     case 0x26:	/* GAMSET */
         if (!s->pm) {
-            s->gamma = ffs(s->param[0] & 0xf) - 1;
+            s->gamma = ctz32(s->param[0] & 0xf);
+            if (s->gamma == 32) {
+                s->gamma = -1; /* XXX: should this be 0? */
+            }
         } else if (s->pm < 0) {
             s->pm = 1;
         }