[S390] cio: move scsw helper functions to header file

All scsw helper functions are very short and usage of them shouldn't
result in function calls. Therefore we move them to a separate header
file.
Also saves a lot of EXPORT_SYMBOLs.

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