ramips: fix ethernet MAC address on Omega2

ramips: fix ethernet MAC address on Omega2

According to https://docs.onion.io/omega2-docs/mac-address.html, 0x28 is
the correct location to read the address on Onion Omega 2(+) devices.

This fixes a regression introduced by commit 77e850fe76e3 ("ramips: tidy up
MAC address setup for Linkit Smart and Omega2"), which was a cleanup that
intended to preserve existing behavior. In my testing with v19.07.7,
however, the MAC address determined from the device tree takes precedence
over the one set by 02_network, so the aforementioned commit actually
changed the behavior.

Signed-off-by: Michael Siegenthaler <msiegen@google.com>
[Adapt patch to nvmem usage]
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>