s390/qeth: add VNICC get/set timeout support

HiperSockets allow configuring so called VNIC Characteristics (VNICC)
that influence how the underlying hardware handles packets. For VNICCs,
additional commands for getting and setting timeouts are available.
Currently, the learning VNICC uses these commands.

* Learning VNICC: If learning is enabled on a qeth device, the device
  learns the source MAC addresses of outgoing packets and incoming
  packets to those learned MAC addresses are received.

For learning, the timeout specifies the idle period in seconds, after
which the underlying hardware removes a learned MAC address again.

This patch adds support for the IPA commands that are required to get
and set the current timeout values for the learning VNIC characteristic.
Also, it introduces the sysfs interface that allows users to configure
the timeout.

Signed-off-by: Hans Wippel <hwippel@linux.vnet.ibm.com>
Reviewed-by: Julian Wiedmann <jwi@linux.vnet.ibm.com>
Signed-off-by: Julian Wiedmann <jwi@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>