[S390] css: move chsc_private to drv_data

Use the subchannels drv_data to access chsc_private
for chsc subchannels.

Note: Also set the drv_data prior to the cio_enable_subchannel
call, since we can receive an interrupt the moment we enable
the subchannel and the IRQ handler relies on this data.

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>