eigrp: Make the eigrp_interface have a prefix instead of a *prefix

eigrp: Make the eigrp_interface have a prefix instead of a *prefix

The prefix data structure was being freed yet still needed in the
future and it's a fundamental part of the eigrp_interface data
structure let's keep it there instead of having it be deleted
and then not.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>