lcs: Return zero from ccwgroup devs set_offline function

A return code of non-zero of the devs ccwgroup set_offline function
leaves the dev in online state. Having done a partly offlining
may leave the dev in unusable state. To make sure the dev is set to
offline, zero is returned. A setup trace entry is written in case
the offlining steps encountered some trouble.

Signed-off-by: Klaus-Dieter Wacker <kdwacker@de.ibm.com>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>