net: ethernet: mediatek: Allow non TRGMII mode with MT7621 DDR2 devices

No reason to error out on a MT7621 device with DDR2 memory when non
TRGMII mode is selected.
Only MT7621 DDR2 clock setup is not supported for TRGMII mode.
But non TRGMII mode doesn't need any special clock setup.

Signed-off-by: René van Dorst <opensource@vdorst.com>
Signed-off-by: David S. Miller <davem@davemloft.net>