Revert "ramips: convert MT7915 EEPROM to NVMEM format"

Revert "ramips: convert MT7915 EEPROM to NVMEM format"

Some MT7915 devices need to load the second part of the eeprom to
work properly. The mt76 driver is not yet ready to read the pre-cal
data via the NVMEM cell. Therefore, partially revert commit to fix
the device probe issue on some devices.

P.S.
Except for D-Link and Ubnt devices, It is still uncertain whether
pre-cal data is required for other devices in the patch.

This partially reverts commit 9ac891f8c44124e931c15f1257347cd8ba311a19.

Fixes: https://github.com/openwrt/openwrt/issues/13700
Signed-off-by: Shiji Yang <yangshiji66@qq.com>