Fix carelessly copy pasted comment

Signed-off-by: Paul Elliott <paul.elliott@arm.com>

library/base64.c

diff --git a/library/base64.c b/library/base64.c
index 267dcc2..edbcfca 100644
--- a/library/base64.c
+++ b/library/base64.c
@@ -78,7 +78,7 @@
 #pragma warning( disable : 4146 )
 #endif
 
-    /* Generate bitmask from condition, mask will either be 0xFFFFFFFF or 0 */
+    /* Generate bitmask from condition, mask will either be 0xFF or 0 */
     unsigned char mask = ( condition | -condition );
     mask >>= 7;
     mask = -mask;