pppd: Fix ether_to_eui64() to prefer devnam ether interface (#206)

pppd: Fix ether_to_eui64() to prefer devnam ether interface (#206)

In case of PPPoE connection, devnam is ethernet interface. So in this case
calculate EUI-64 IPV6CP identifier from the PPPoE ethernet interface
instead from the first ethernet interface in the system.

This would ensure that each PPPoE connection would have unique IPv6 link
local address calculated from the interface MAC address on which is PPPoE
going to be established.

Currently IPv6 link local address is always calculated from the MAC address
of the first ethernet interface and therefore all active PPPoE connections
have by default same (non unique) IPv6 link local address.

Signed-off-by: Pali Rohár <pali@kernel.org>