staging: comedi: rename dev parameter of device attribute functions

Most of the comedi core uses the identifier `dev` to point to a `struct
comedi_device`.  The device sysfs attribute functions such as
`show_max_read_buffer_kb()` use the parameter id `dev` to point to a
`struct device`.  Rename the parameter to `csdev` for "class device" for
consistency with the functions that call `device_create()` to create
these class devices (`comedi_alloc_board_minor()` and
`comedi_alloc_subdevice_minor()`).

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>