r8169: improve DASH support

Instead of doing the full DASH check each time r8168_check_dash() is
called, let's do it once in probe and store DASH capabilities in a
new rtl8169_private member.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>