ethtool: Fix ptest compile

buildtest-TESTS is a phony target and does nothing which results in a
do_install error since the tests aren't built. Since there isn't
a suitable make target but the number of tests are small, hardcode
the two to build to unbreak the build when ptest is enabled.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>