luci-mod-network: allow device declarations and device configs to coexist

It is legal to have two device sections referring to the same netdev if one
section is a declaration (a section setting option type) and the other is
a configuration (a section not specifying a type but matching an existing
netdev).

Support this case in LuCI since it might be required for some complex
device setups.

Additionally, fix the device type determination for device configuration
sections without type, those should be treated as ethernet (a.k.a.
simple device) configuration instead of falling back to the underlying
netdev device type.

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