Unannounced modifications (OpenWrt PR 13780) in uci network.lan.ipaddr broke tvheadend init script.
This commit updates the init script to cut the subnet from the ip address reported by uci.
Bug report: #26681
Reported-by: @DonKult
Tested-by: @DonKult
Signed-off-by: Marius Dinu <m95d+git@psihoexpert.ro>