base-files: fix ucidef_set_interface() protocol selection

base-files: fix ucidef_set_interface() protocol selection

The previous refactoring of ucidef_set_interface() removed the protocol
selection heuristic which breaks the networking defaults for the majority
of boards.

Re-add the protocol selection and rename two bad "proto" references to
the expected "protocol" value.

Fixes: 85048a9c1f ("base-files: rework _ucidef_set_interface to be more generic")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>
(backported from 7e664b7c2dc00006ba29bf947cf177b5bccdc47d)