[S390] qdio_get_micros return value.

qdio_get_micros is supposed to return microseconds. The get_clock()
return value needs to be shifted by 12 to get to microseconds.

Signed-off-by: Frank Pavlic <fpavlic@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>