ifconfig: fix logical error in interface matching for '-l ether'

This affects only ifconfig(8) compiled WITHOUT_NETLINK_SUPPORT, which
is not the default.

Reviewed by: melifaro
Differential Revision: https://reviews.freebsd.org/D41584
Fixes: d1cd0344f7b7d81beda04c3cb8cfee99351c3eb8

(cherry picked from commit 24a81a968d21b3eef1a0a2d786859b274546bd34)