net: stmmac: eswin: clear TXD and RXD delay registers during initialization

net: stmmac: eswin: clear TXD and RXD delay registers during initialization

Clear the TXD and RXD delay control registers during EIC7700 DWMAC
initialization.

These registers may retain values programmed by the bootloader. If left
unchanged, residual delays can alter the effective RGMII timing seen by
the MAC and override the configuration described by the device tree.

This may violate the expected RGMII timing model and can cause link
instability or prevent the Ethernet controller from operating correctly.

Explicitly clearing these registers ensures that the MAC delay settings
are determined solely by the kernel configuration.

The corresponding register offsets are optional, and the registers are
only cleared when the offsets are provided in the device tree.

Fixes: ea77dbbdbc4e ("net: stmmac: add Eswin EIC7700 glue driver")
Signed-off-by: Zhi Li <lizhi2@eswincomputing.com>
Link: https://patch.msgid.link/20260518022137.464-1-lizhi2@eswincomputing.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>