luci-app-https_dns_proxy: fix datatype of subnet address option

The datatype ip4prefix only accept numbers from 0 to 32, not subnet/mask
(i.e. 192.168.1.0/24), this affects option "-e".

https://github.com/openwrt/luci/wiki/Datatypes#ip4prefix the description
is wrong, according to src
http://luci.subsignal.org/trac/browser/luci/branches/luci-0.10/libs/web/luasrc/cbi/datatypes.lua#L76

Submitted-by: alphakk <kkspaces@mail.com>
[slightly reword commit message]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>