mcommphy: Set the 10/100 TX delay of the YT8511

[manu]

Summary

The TX clock delay of the YT8511 is split in two registers. The delay used
in gigabit mode is in the clock gating page and was already set from the
RGMII mode, but the one used in the 10/100 modes is in the next page and was
left at its reset value of 2ns. With a phy-mode that does not ask for a TX
delay in the PHY, the MAC provides it and the PHY adds another 2ns in the
10/100 modes.

Set it like the gigabit one, like Linux does.

Tested on a Quartz64 Model A, gigabit and 100Mbit links.