net: stmmac: dwmac-rk: Check platform-specific ops

Add a check for non-null struct rk_gmac_ops for the
configured PHY interface mode, failing if unsupported.

Signed-off-by: David Wu <david.wu@rock-chips.com>
[Ezequiel: Refactor so it fails if unsupported]
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: David S. Miller <davem@davemloft.net>