ifconfig: fix 'ifconfig -l ether'

When matching interfaces for being Ethernet, use same trick that
the link module does - pass if_type through convert_iftype().
That restores historicaly behaviour of listing lagg(4) ports.

Reviewed by: melifaro
Differential Revision: https://reviews.freebsd.org/D41585

(cherry picked from commit 31016aa0be6dbf7556a91dda14cfe1ff0dcf933b)