vendorcode/amd: Fix non-terminating loop

Code is copied from agesa/common's amdlib.c.
Things can probably be deduplicated.

Change-Id: I9c8adab5db7e9fd41aecc522136dfa705c1e2ee6
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Found-by: Coverity Scan #1229662
Reviewed-on: https://review.coreboot.org/17834
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Martin Roth <martinroth@google.com>
Reviewed-by: Marshall Dawson <marshalldawson3rd@gmail.com>
Tested-by: build bot (Jenkins)

src/vendorcode/amd/pi/Lib/amdlib.c

@@ -381,11 +381,17 @@ LibAmdBitScanReverse (
   IN       UINT32 value
 )
 {
-  UINTN Index;
-  for (Index = 31; Index >= 0; Index--){
-      if (value & (1 << Index)) break;
-  }
-  return (UINT8) Index;
+  uint8_t bit = 31;
+  do {
+    if (value & (1 << 31))
+      return bit;
+
+    value <<= 1;
+    bit--;
+
+  } while (value != 0);
+
+  return 0xFF; /* Error code indicating no bit found */
 }
 
 AMDLIB_OPTIMIZE