staging: comedi: fixed multiple line dereference

Fixed multiple line dereference for &cmd->scan_begin_arg.

Signed-off-by: Artur Lorincz <larturus@yahoo.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>