s390x: STFLE operates on doublewords

STFLE operates on doublewords, not bytes. Passing in "256" resulted in
some ignored bits getting set. Not bad, but also not clean.

Let's just convert our stfle handling code to operate on doublewords.

Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Message-Id: <20200401163305.31550-1-david@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>