qeth: remove needless IPA-commands in offline

If a qeth device is set offline, data and control subchannels are
cleared, which means removal of all IP Assist Primitive settings
implicitly. There is no need to delete those settings explicitly.
This patch removes all IP Assist invocations from offline.

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>