net/phy/vitesse: Rework RGMII skew configuration for VSC8601

The VSC8601 config tried to add an RGMII skew based on #defines that
no config defines. That's quite an ugly way to do it. Since the skew
is only needed on RGMII interfaces, check the interface mode at
runtime, and apply the settings accordingly.

Tested on custom board with AM3352 SOC and VSC801 PHY.

Signed-off-by: Alexandru Gagniuc <alex.g@adaptrum.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>