It's missing a hyphen present in every other LED from the set. Set it to
"green:signal-strength-4".

Signed-off-by: Lech Perczak <lech.perczak@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/17503


Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>