qeth: serialize sysfs-triggered device configurations

This patch serializes device removal and other sysfs-triggered
configurations by moving removal of sysfs-attributes to the beginning
of the remove functions. And it serializes online/offline setting
and discipline-switching (causing reestablishing of the net_device)
by making use of a new discipline mutex.

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