Fix EC API usage due to change in mbed-TLS

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/boot/bootutil/src/image_ec.c b/boot/bootutil/src/image_ec.c
index ee2b594..6c3d5b2 100644
--- a/boot/bootutil/src/image_ec.c
+++ b/boot/bootutil/src/image_ec.c
@@ -66,7 +66,7 @@
         return -4;
     }
 
-    if (mbedtls_ecp_group_load_secp224r1(&ctx->grp)) {
+    if (mbedtls_ecp_group_load(&ctx->grp, MBEDTLS_ECP_DP_SECP224R1)) {
         return -5;
     }