r8169: use helper rtl_hw_aspm_clkreq_enable also in rtl_hw_start_8168g_2

One place in the driver was left where the open-coded functionality
hasn't been replaced with helper rtl_hw_aspm_clkreq_enable yet.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>