Fix documentation for mbedtls_gcm_finish()

Fix implementation and documentation missmatch for the function
arguments to mbedtls_gcm_finish(). Also, removed redundant if condition
that always evaluates to true.
diff --git a/ChangeLog b/ChangeLog
index ed6f221..d64ce68 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -17,6 +17,8 @@
    * Fixed the sample applications gen_key.c, cert_req.c and cert_write.c for
      builds where the configuration POLARSSL_PEM_WRITE_C is not defined. Found
      by inestlerode. #559.
+   * Fix documentation and implementation missmatch for function arguments of
+     mbedtls_gcm_finish(). Found by cmiatpaar.
 
 = mbed TLS 1.3.17 branch 2016-06-28