r8169: simplify and improve check for dash

r8168_check_dash() returns false anyway for all chip versions not
supporting dash. So we can simplify the check conditions.

In addition change the check functions to return bool instead of int,
because they actually return a bool value.

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