test: Rename interfaces in ptp clock test

This change is required to allow correct tests operation after changing
the definition of linker script placement rule (to be more strict and
using the device name).

The struct net_if interfaces have been renamed to preserve correct
order.
Moreover, the iface_cb() function now immediately returns for
interfaces, which are not supposed to be used in this tests - for
example ones already defined on board on which the test is run.

Signed-off-by: Lukasz Majewski <lukma@denx.de>