de2104x: support for systems lacking cache coherence

Add a configurable Descriptor Skip Length for systems that lack cache
coherence.

(akpm: I think this should be done as a module parameter, not a
compile-tinme option)

Signed-off-by: Risto Suominen <Risto.Suominen@gmail.com>
Cc: Grant Grundler <grundler@parisc-linux.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>