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
@@ -506,7 +506,7 @@ void mbedtls_ecp_group_init( mbedtls_ecp_group *grp )
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 );
