s390x: css: msch, enable test

A second step when testing the channel subsystem is to prepare a channel
for use.
This includes:
- Get the current subchannel Information Block (SCHIB) using STSCH
- Update it in memory to set the ENABLE bit and the specified ISC
- Tell the CSS that the SCHIB has been modified using MSCH
- Get the SCHIB from the CSS again to verify that the subchannel is
  enabled and uses the specified ISC.
- If the command succeeds but subchannel is not enabled or the ISC
  field is not as expected, retry a predefined retries count.
- If the command fails, report the failure and do not retry, even
  if cc indicates a busy/status pending as we do not expect this.

This tests the MSCH instruction to enable a channel successfully.
Retries are done and in case of error, and if the retries count
is exceeded, a report is made.

Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
Acked-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <1594887809-10521-9-git-send-email-pmorel@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>