editor: disable MTU spin button in Ethernet page for PPPoE connections

The MTU property in the wired setting has no effect for PPPoE
connections, as the MTU for the PPP interface is controlled by the
ppp.mtu properties.

Disable the MTU spin button if the connection is a PPPoE one since
it's confusing for users.

https://bugzilla.redhat.com/show_bug.cgi?id=1385198