Remove |num_rounds| argument from chacha_core.

The function was hard-coded to 20 rounds already so the argument was
already useless. Thanks to Huzaifa Sidhpurwala for noticing.

Change-Id: I5f9d6ca6d46c6ab769b19820f8f918349544846d

crypto/chacha/chacha_generic.c

diff --git a/crypto/chacha/chacha_generic.c b/crypto/chacha/chacha_generic.c
index 1e5b70d..b5daed2 100644
--- a/crypto/chacha/chacha_generic.c
+++ b/crypto/chacha/chacha_generic.c
@@ -56,8 +56,7 @@
 
 /* chacha_core performs |num_rounds| rounds of ChaCha20 on the input words in
  * |input| and writes the 64 output bytes to |output|. */
-static void chacha_core(uint8_t output[64], const uint32_t input[16],
-                        int num_rounds) {
+static void chacha_core(uint8_t output[64], const uint32_t input[16]) {
   uint32_t x[16];
   int i;
 
@@ -122,7 +121,7 @@
       todo = in_len;
     }
 
-    chacha_core(buf, input, 20);
+    chacha_core(buf, input);
     for (i = 0; i < todo; i++) {
       out[i] = in[i] ^ buf[i];
     }