sd: convert sd_normal_command() ffs(3) call to ctz32()

ffs() cannot be replaced with ctz32() when the argument might be zero,
because ffs(0) returns 0 while ctz32(0) returns 32.

The ffs(3) call in sd_normal_command() is a special case though.  It can
be converted to ctz32() + 1 because the argument is never zero:

  if (!(req.arg >> 8) || (req.arg >> (ctz32(req.arg & ~0xff) + 1))) {
      ~~~~~~~~~~~~~~~
            ^--------------- req.arg cannot be zero

Cc: Markus Armbruster <armbru@redhat.com>
Cc: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1427124571-28598-7-git-send-email-stefanha@redhat.com
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

hw/sd/sd.c

@@ -796,8 +796,9 @@ static sd_rsp_type_t sd_normal_command(SDState *sd,
             sd->vhs = 0;
 
             /* No response if not exactly one VHS bit is set.  */
-            if (!(req.arg >> 8) || (req.arg >> ffs(req.arg & ~0xff)))
+            if (!(req.arg >> 8) || (req.arg >> (ctz32(req.arg & ~0xff) + 1))) {
                 return sd->spi ? sd_r7 : sd_r0;
+            }
 
             /* Accept.  */
             sd->vhs = req.arg;