r8169: allow setting irq coalescing if link is down

So far we can not configure irq coalescing when link is down. Allow the
user to do this, and assume that he wants to configure irq coalescing
for highest speed. Otherwise the irq rate is low enough anyway.

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