r8169: improve rtl8169_mark_to_asic

Let the compiler decide about inlining, and as confirmed by Eric it's
better to use WRITE_ONCE here to ensure that the descriptor ownership
is transferred to NIC immediately.

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