s390/pfault: cleanup inline assemblies

Cleanup the pfault inline assemblies:
- Use symbolic names for operands
- Add extra linebreaks, and whitespace to improve readability

In addition, change __pfault_init() to return -EOPNOTSUPP in case of
an exception, and don't return a made up valid diag 258 return value
(aka "8").
This allows to simplify the inline assembly, and makes debugging
easier, in case something is broken.

Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>