dnsmasq: Fix splitting hostid for DHCPv6 static leases

Correct splitting the 32-bit 'hostid' value to two 16-bit hexadecimal
values. Previously, the lower 16-bit value was truncated to an 8-bit
value, which would result in hostid values 100 and 200 both to be set
to [::0:0] instead of [::0:100] and [::0:200] respectively.

Signed-off-by: Arjen de Korte <build+lede@de-korte.org>