drivers: ethernet: Set the context iface ptr to main interface

If the Ethernet driver has VLAN enabled (only native_posix, mcux
or gmac has VLAN supported), then the iface pointer in ethernet
context should contain the main network interface. This is needed
so that the interface will get link address set to it properly.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>