Staging: comedi: kcomedilib: replace deprecated simple_strtoul() with kstrtouint()

Since simple_strtoul() has been deprecated, replace it with kstrtouint().
Also, since return code checking for this new function is enforced, add a
check to ensure that the conversion has succeeded.

Signed-off-by: Chase Southwood <chase.southwood@yahoo.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>