cipher: handle ChaCha20 as a stream cipher

That's what it is. So we shouldn't set a block size != 1.

While at it, move call to chachapoly_update() closer to the one for GCM, as
they are similar (AEAD).

include/mbedtls/cipher.h

@@ -193,7 +193,7 @@ enum {
 /** Maximum length of any IV, in Bytes. */
 #define MBEDTLS_MAX_IV_LENGTH      16
 /** Maximum block size of any cipher, in Bytes. */
-#define MBEDTLS_MAX_BLOCK_LENGTH   64
+#define MBEDTLS_MAX_BLOCK_LENGTH   16
 
 /**
  * Base cipher information (opaque struct).