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 57a5735..7782470 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -45,6 +45,8 @@
    * Fix mbedtls_x509_get_sig() to update the ASN1 type in the mbedtls_x509_buf
      data structure until after error checks are successful. Found by
      subramanyam-c.
+   * Fix documentation and implementation missmatch for function arguments of
+     mbedtls_gcm_finish(). Found by cmiatpaar.
 
 Changes
    * Extended test coverage of special cases, and added new timing test suite.