selftests: drv-net: Move _set_ethtool_feat() into lib

Move the _set_ethtool_feat() helper from gro_lib.py into lib, so that it
can be reused by the VLAN test added in the next commit. Drop the leading
underscore, now that the helper is exported.

Signed-off-by: Ovidiu Panait <ovidiu.panait.rb@renesas.com>
Reviewed-by: Nicolai Buchwitz <nb@tipi-net.de>
Link: https://patch.msgid.link/20260918112529.96039-2-ovidiu.panait.rb@renesas.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>