tls: use already existing symbols to size the buffer for XXDḦ peer key

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/library/ssl_misc.h b/library/ssl_misc.h
index 2b0b7f1..c08f5e1 100644
--- a/library/ssl_misc.h
+++ b/library/ssl_misc.h
@@ -757,24 +757,12 @@
 #endif /* !MBEDTLS_USE_PSA_CRYPTO &&
           MBEDTLS_KEY_EXCHANGE_SOME_ECDH_OR_ECDHE_1_2_ENABLED */
 
-#if defined(PSA_WANT_ALG_ECDH) && defined(PSA_WANT_ALG_FFDH)
-#if (MBEDTLS_PSA_MAX_FFDH_PUBKEY_LENGTH >= MBEDTLS_PSA_MAX_EC_PUBKEY_LENGTH)
-#define SSL_XXDH_PSA_PEERKEY_SIZE MBEDTLS_PSA_MAX_FFDH_PUBKEY_LENGTH
-#else
-#define SSL_XXDH_PSA_PEERKEY_SIZE MBEDTLS_PSA_MAX_EC_PUBKEY_LENGTH
-#endif
-#elif defined(PSA_WANT_ALG_ECDH)
-#define SSL_XXDH_PSA_PEERKEY_SIZE MBEDTLS_PSA_MAX_EC_PUBKEY_LENGTH
-#else
-#define SSL_XXDH_PSA_PEERKEY_SIZE MBEDTLS_PSA_MAX_FFDH_PUBKEY_LENGTH
-#endif
-
 #if defined(MBEDTLS_KEY_EXCHANGE_SOME_XXDH_ANY_PSA_ENABLED)
     psa_key_type_t xxdh_psa_type;
     size_t xxdh_bits;
     mbedtls_svc_key_id_t xxdh_psa_privkey;
     uint8_t xxdh_psa_privkey_is_external;
-    unsigned char xxdh_psa_peerkey[SSL_XXDH_PSA_PEERKEY_SIZE];
+    unsigned char xxdh_psa_peerkey[PSA_EXPORT_PUBLIC_KEY_MAX_SIZE];
     size_t xxdh_psa_peerkey_len;
 #endif /* MBEDTLS_KEY_EXCHANGE_SOME_XXDH_ANY_PSA_ENABLED */