s390/cmma: reuse kstrtobool for option value parsing

"cmma" option setup already recognises some textual values. Yet kstrtobool
is a more common way to parse boolean values, reuse it to unify option
value parsing behavior and simplify code a bit.

While at it, __setup value parsing callbacks are expected to return
1 when an option is recognized, and returning any other value won't
trigger any error message currently, so simply return 1.

Reviewed-by: Philipp Rudo <prudo@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>