s390/sclp_sdias: Add missing break and "fall through"

If the last block of the HSA is read, EVSTATE_ALL_STORED is returned
by SCLP. Because of a missing break in the switch statement two trace
entries are written in this case: "all stored" and "part stored".

This patch adds the missing break and also adds a "fall through"
comment to improve the readability.

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Reported-by: David A Gilbert <DavidAGilbert@uk.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>