Use MD not low-level sha256/512 in TLS

Same reasoning as in previous commit.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/library/ssl_misc.h b/library/ssl_misc.h
index 7385c6e..7abbffb 100644
--- a/library/ssl_misc.h
+++ b/library/ssl_misc.h
@@ -905,14 +905,14 @@
 #if defined(MBEDTLS_USE_PSA_CRYPTO)
     psa_hash_operation_t fin_sha256_psa;
 #else
-    mbedtls_sha256_context fin_sha256;
+    mbedtls_md_context_t fin_sha256;
 #endif
 #endif
 #if defined(MBEDTLS_HAS_ALG_SHA_384_VIA_MD_OR_PSA_BASED_ON_USE_PSA)
 #if defined(MBEDTLS_USE_PSA_CRYPTO)
     psa_hash_operation_t fin_sha384_psa;
 #else
-    mbedtls_sha512_context fin_sha384;
+    mbedtls_md_context_t fin_sha384;
 #endif
 #endif