r8169: factor out handling rtl8169_stats

Factor out handling the private packet/byte counters to new
functions rtl_get_priv_stats() and rtl_inc_priv_stats().

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>