luci-mod-network: make device selector dependent on protocol (again)

This commit effectively reverts the change made with
907b4222 ("luci-mod-network: don't hide "Device" on protocol change").

Floating tunnel protocols such as 6in4, plain PPP over modem device,
VPNC etc. do not have any layer 2 device at all, for such protocols the
device selector should be hidden.

Also swap back the incorrect option order introduced with commit
b7f3cf66

 ("luci-mod-network: drop support for *adding* legacy bridges").

Since device depends on proto, it should come after the protocol selection,
not before.

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