s390/sclp: use kstrobool() to parse sclp_con_drop parameter

Use kstrobool() to parse sclp_con_drop parameter. This way handling of
valid values for the sclp_con_drop parameter is identical to the
con3215_drop parameter. In particular this allows to pass values like
"yes" and "true", which was not possible before.

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>