s390/qeth: move common ioctl handling to core

There's a number of layer-independent ioctls that we can handle
in core, and reduce code duplication. For layer-specific ioctls,
add a do_ioctl() discipline hook.

Signed-off-by: Julian Wiedmann <jwi@linux.vnet.ibm.com>
Acked-by: Ursula Braun <ubraun@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>