Make variables const in write_uniform() and read_uniform()

BUG=aomedia:448

Change-Id: Ie02581a20ca12b24d0a5719a1d9220fb20e25ff7

av1/encoder/bitstream.c

diff --git a/av1/encoder/bitstream.c b/av1/encoder/bitstream.c
index 8f0342b..bd654fb 100644
--- a/av1/encoder/bitstream.c
+++ b/av1/encoder/bitstream.c
@@ -98,8 +98,8 @@
 
 #if CONFIG_EXT_INTRA || CONFIG_FILTER_INTRA || CONFIG_PALETTE
 static INLINE void write_uniform(aom_writer *w, int n, int v) {
-  int l = get_unsigned_bits(n);
-  int m = (1 << l) - n;
+  const int l = get_unsigned_bits(n);
+  const int m = (1 << l) - n;
   if (l == 0) return;
   if (v < m) {
     aom_write_literal(w, v, l - 1);