rockchip: don't set static ula_prefix

[Howard20181]

Randomize it like

friendlywrt/package/base-files/files/etc/uci-defaults/12_network-generate-ula

Lines 3 to 10 in 96e1d67

	r1=$(dd if=/dev/urandom bs=1 count=1 |hexdump -e '1/1 "%02x"')
	r2=$(dd if=/dev/urandom bs=2 count=1 |hexdump -e '2/1 "%02x"')
	r3=$(dd if=/dev/urandom bs=2 count=1 |hexdump -e '2/1 "%02x"')
	uci -q batch <<-EOF >/dev/null
	set network.globals.ula_prefix=fd$r1:$r2:$r3::/48
	commit network
	EOF