Use PSA-neutral function for availability check

We just want to check if this hash is available, and the check is
present in builds both with PSA and without it. The function we were
using is only present in builds with PSA, so it wasn't appropriate.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/library/rsa.c b/library/rsa.c
index 950d8e9..8126ae9 100644
--- a/library/rsa.c
+++ b/library/rsa.c
@@ -478,7 +478,7 @@
     if ((padding == MBEDTLS_RSA_PKCS_V21) &&
         (hash_id != MBEDTLS_MD_NONE)) {
         /* Just make sure this hash is supported in this build. */
-        if (mbedtls_md_psa_alg_from_type(hash_id) == PSA_ALG_NONE) {
+        if (mbedtls_md_info_from_type(hash_id) == NULL) {
             return MBEDTLS_ERR_RSA_INVALID_PADDING;
         }
     }