[etherswitch] simplify kernconf for recently added etherswitch drivers

This simple patch adds e6060sw, adm6996fc and ksz8995ma into conf/files.

Submitted by: Hiroki Mori <yamori813@yahoo.co.jp>
Reviewed by: adrian, mizhka
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D9255