s390/sclp: allow to change sclp_console_drop during runtime

Make sclp's con_drop sysfs attribute also writable, and allow to
change its value during runtime. This way handling of sclp console
drop handling is consistent with the 3215 device driver.

Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>