phy_init: don’t rewrite valid calibration data

phy_init: don’t rewrite valid calibration data

In the default PHY init routine, calibration data is loaded from NVS.
Most of the time the incremental changes to calibration will be fairly
small, so we don’t need to rewrite the existing calibration data stored
in the NVS.

Possible enhancement to be done in the future: expose a function in PHY
library to tell how big was the change in calibration data. If the
change was significant, then calibration data stored in NVS should be
updated.