net: dsa: rtl8366rb: Support port 4 (WAN)

The totally undocumented IO mode needs to be set to enumerator
0 to enable port 4 also known as WAN in most configurations,
for ordinary traffic. The 3 bits in the register come up as
010 after reset, but need to be set to 000.

The Realtek source code contains a name for these bits, but
no explanation of what the 8 different IO modes may be.

Set it to zero for the time being and drop a comment so
people know what is going on if they run into trouble. This
"mode zero" works fine with the D-Link DIR-685 with
RTL8366RB.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>