Using stsch on schids with ssid != 0 can lead to an operand
exception. Use stsch_err to handle potential exceptions
if we fail to reenable mss after hibernation.

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