s390/ism: Cleanups

Reworked signature of the function to retrieve the system EID: No plausible
reason to use a double pointer. And neither to pass in the device as an
argument, as this identifier is by definition per system, not per device.
Plus some minor consistency edits.

Signed-off-by: Stefan Raspl <raspl@linux.ibm.com>
Signed-off-by: Wenjia Zhang &lt; <wenjia@linux.ibm.com>
Reviewed-by: Tony Lu <tonylu@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>