[S390] cpcmd: fix inline assembly usage.

After assigning values to specific registers memset was called. This
may clobber the contents of the used registers.
To solve this extract the two used inline assemblies into small
functions that don't call any functions.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>