generate/derive key ext: pass method_data_length rather than method_length

Instead of passing the size of the whole structure, just pass the data
length and let the implementation worry about adding the size of the
structure. The intent with passing the structure size was to allow
the client code in a client-server implementation to know nothing
about the structure and just copy the bytes to the server. But that was not
really a useful consideration since the application has to know the
structure layout, so it has to be available in the client implementation's
headers. Passing the method data length makes life simpler for everyone by
not having to worry about possible padding at the end of the structure, and
removes a potential error condition
(method_length < sizeof(psa_key_generation_method_t)).

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

include/psa/crypto_struct.h

@@ -233,7 +233,7 @@ struct psa_key_generation_method_s {
  *
  * Calling psa_generate_key_ext() or psa_key_derivation_output_key_ext()
  * with `method=PSA_KEY_GENERATION_METHOD_INIT` and
- * `method_length=sizeof(psa_key_generation_method_t)` is equivalent to
+ * `method_data_length == 0` is equivalent to
  * calling psa_generate_key() or psa_key_derivation_output_key()
  * respectively.
  */