s390/kdump: cleanup do_start_kdump() prototype and usage

Avoid unnecessary run-time and compile-time type
conversions of do_start_kdump() function return
value and parameter.

Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>