base-files: fix wrong ucidef_set_network_device_mac network-device entry

base-files: fix wrong ucidef_set_network_device_mac network-device entry

The ucidef_set_network_device_* functions in uci-defaults.sh disagree
on whether to use "network-device" or "network_device" in board.json.
With the additional caveat that jshn will translate hyphens (-) into
underscores (_). This casues problems in netifd which expected
"network_device" causing boards which depend on assigning MACs in
board.json via uci-defaults.sh (or jshn in general) to fail.

This commit addresses the issue by using network_device in
uci-defaults.sh.

The bug was uncovered in the forums here:
https://forum.openwrt.org/t/support-for-rtl838x-based-managed-switches/57875/2596

This was exposed by commit 4ebba8a05d09 ("realtek: add support for HPE
1920-8g-poe+") where the board_config_load call from 03_gpio introduced
the key normalization by jshn.

Fixes: 9290539ca9c7 ("base-files: allow setting device and bridge macs")
Tested-by: Stijn Segers <foss@volatilesystems.org>
Signed-off-by: Michael 'ASAP' Weinrich <michael@a5ap.net>
[ improve commit title, description and fix wrong Tested-by tag ]
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>