s390/cio: unregister the subchannel while purging

The cio_ignore list is used to create and maintain the list of devices
which is to be ignored by Linux. During boot-time, this list is adjusted
and accommodate all the devices which are configured on the HMC
interface. Once these devices are accessible, they are then available to
Linux and set online.

cio_ignore purge function should align with this functionality. But
currently, the subchannel associated with the offline-devices are not
unregistered during purge. Add an explicit subchannel-unregister function
in the purge_fn callback.

Signed-off-by: Vineeth Vijayan <vneethv@linux.ibm.com>
Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>