staging: comedi: comedi_fops: rename the __user *cmd param in do_cmd_ioctl

This parameter is actually the unsigned long arg passed in the ioctl.
comedi_unlocked_ioctl() casts it as a (struct comedi_cmd __user *)
when calling do_cmd_ioctl(). Rename the variable to keep this clear.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Acked-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>