staging: comedi: comedi_fops: Replace deprecated strict_strtoul with kstrtouint

As reported by checkpatch.pl strict_strtoul should be replaced. It was
replaced with kstrtouint since async->max_bufsize is an unsigned int
anyway.

Signed-off-by: Florian Schmaus <fschmaus@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>