s390x/event-facility.c: remove unneeded labels

s390x/event-facility.c: remove unneeded labels

'out' label from write_event_mask() and write_event_data()
can be replaced by 'return'.

The 'out' label from read_event_data() can also be replaced.
However, as suggested by Cornelia Huck, instead of simply
replacing the 'out' label, let's also change the code flow
a bit to make it clearer that sccb events are always handled
regardless of the mask for unconditional reads, while selective
reads are handled if the mask is valid.

CC: Cornelia Huck <cohuck@redhat.com>
CC: Thomas Huth <thuth@redhat.com>
CC: Halil Pasic <pasic@linux.ibm.com>
CC: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20200108144607.878862-1-danielhb413@gmail.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>