ath79: meraki-mr18: fix MAC address offset after NVMEM switch

ath79: meraki-mr18: fix MAC address offset after NVMEM switch

The following commit: 27a673916c35 ("ath79: mr18: use nvmem for MACs")
switched MR18 to use NVMEM subsystem for setting MAC addresses, however
it missed the offset in use. Previously 102 (decimal) was used, but in
device tree 0x102 was used, but the correct value is 0x66.

This was found while reviewing code for Z1 port, which shares the MAC
address source.
Replace the offset with the correct one of 0x66.

Fixes: 27a673916c35 ("ath79: mr18: use nvmem for MACs")
Signed-off-by: Lech Perczak <lech.perczak@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/23486
Signed-off-by: Robert Marko <robimarko@gmail.com>