s390/ap: clarify AP scan bus related functions and variables

This patch tries to clarify the functions and variables
around the AP scan bus job. All these variables and
functions start with ap_scan_bus and are declared in
one place now.

No functional changes in this patch - only renaming and
move of code or declarations.

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Holger Dengler <dengler@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>