strtobool() does check for NULL parameter already. No need to repeat.

While here, switch to kstrtobool() and unshadow actual error code
(which is still -EINVAL).

No functional change intended.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>