luci-app-wol: use uci etherwake default interface

Use uci config 'etherwake.setup.interface' to default WOL interface
selection.

On some devices the lan interface is not the first item in the drop down
interface selection list.  Configuring this parameter allows the user to
set a preferred interface to be used/selected.

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>