luci-app-firewall: adjust drop_invalid default (#1068)

Since firewall3 commit b33f78371e7c7b6a131c2b6c01673cbd4b3c13d1 the
drop_invalid option is off by default.

Adjust LuCI view to properly handle the changed semantics.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>