Fix IAR warning

The IAR compiler doesn't like it when we assign an int to an enum variable.

"C:\builds\ws\mbedtls-restricted-pr\library\ecp.c",509  Error[Pe188]:
          enumerated type mixed with another type
diff --git a/library/ecp.c b/library/ecp.c
index 52d6d9c..bbc1ff0 100644
--- a/library/ecp.c
+++ b/library/ecp.c
@@ -506,7 +506,7 @@
     if( grp == NULL )
         return;
 
-    grp->id = 0;
+    grp->id = MBEDTLS_ECP_DP_NONE;
     mbedtls_mpi_init( &grp->P );
     mbedtls_mpi_init( &grp->A );
     mbedtls_mpi_init( &grp->B );