qeth: bridgeport support - address notifications

Introduce functions to enable and disable bridgeport address
notification feature, sysfs attributes for access to these
functions from userspace, and udev events emitted when a host
joins or exits a bridgeport-enabled HiperSocket channel.

Signed-off-by: Eugene Crosser <eugene.crosser@ru.ibm.com>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Reviewed-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>