net: stmmac: eswin: correct RGMII delay granularity to 20 ps

net: stmmac: eswin: correct RGMII delay granularity to 20 ps

The EIC7700 MAC implements programmable RGMII delay adjustment with a
granularity of 20 ps per hardware step.

The driver previously converted rx-internal-delay-ps and
tx-internal-delay-ps values using a 100 ps step size, resulting in
incorrect delay programming.

Update the conversion to use the correct 20 ps granularity so the
programmed delay matches the values described 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/20260518022156.484-1-lizhi2@eswincomputing.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>