qeth: Cleanup for cast-type determination.

Clear separation of cast-type determination (send path) for layer-2
resp. layer-3. Allowing to have inline functions for qeth layer-
discipline.

Signed-off-by: Klaus-Dieter Wacker <kdwacker@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>