luci-proto-wireguard: fix fwmark validation to allow 32 bit values

The iptables mark field is 32 bits wide, which is 4 bytes and so 8 hex
characters. Fix the fwmark validation to allow 8 characters in the hex
string.

Fixes: #5098
Suggested-by: Robert <32970961+differentblue@users.noreply.github.com>
Signed-off-by: Jo-Philipp Wich <jo@mein.io>