Update guard for ecp

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/library/psa_crypto.c b/library/psa_crypto.c
index 9ef3f1e..456d4e3 100644
--- a/library/psa_crypto.c
+++ b/library/psa_crypto.c
@@ -351,7 +351,7 @@
             return PSA_ERROR_INSUFFICIENT_ENTROPY;
 #endif
 
-#if defined(MBEDTLS_ECP_C)
+#if defined(MBEDTLS_ECP_LIGHT)
         case MBEDTLS_ERR_ECP_BAD_INPUT_DATA:
         case MBEDTLS_ERR_ECP_INVALID_KEY:
             return PSA_ERROR_INVALID_ARGUMENT;