EVP: legacy_ctrl_to_param() to handle provider side keys

EVP: legacy_ctrl_to_param() to handle provider side keys

There was one spot where this function would look at ctx->pmeth
directly to determine if it's for RSASSA-PSS, which fails when
presented with an EVP_PKEY_CTX holding a provider side key.
Switching to use EVP_PKEY_is_a() should make things better.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/11501)