mbedtls_ecp_gen_privkey_mx: rename n_bits to high_bit

For Montgomery keys, n_bits is actually the position of the highest
bit and not the number of bits, which would be 1 more (fence vs
posts). Rename the variable accordingly to lessen the confusion.

No semantic change.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/library/ecp_invasive.h b/library/ecp_invasive.h
index 2895b19..eeb4305 100644
--- a/library/ecp_invasive.h
+++ b/library/ecp_invasive.h
@@ -57,7 +57,7 @@
  * \note            - [Curve25519] http://cr.yp.to/ecdh/curve25519-20060209.pdf
  *                  - [RFC7748] https://tools.ietf.org/html/rfc7748
  *
- * \p n_bits        The position of the high-order bit of the key to generate.
+ * \p high_bit      The position of the high-order bit of the key to generate.
  *                  This is the bit-size of the key minus 1:
  *                  254 for Curve25519 or 447 for Curve448.
  * \param d         The randomly generated key. This is a number of size