s390/fpu: various coding style changes

Address various checkpatch warnings, adjust whitespace, and try to increase
readability. This is just preparation, in order to avoid that subsequent
patches contain any distracting drive-by coding style changes.

Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>