s390/debug: remove module_exit function / move EXPORT_SYMBOLs

debug.c is not a module, so remove the module_exit function, since it
will never be called.
Also move the EXPORT_SYMBOL statements to the functions they belong to.

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