r8169: use standard debug output functions

I see no need to define a private debug output symbol, let's use the
standard debug output functions instead. In this context also remove
the deprecated PFX define.

The one assertion is wrong IMO anyway, this code path is used also
by chip version 01.

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