staging: comedi: drivers do not need to reset the async->cur_chan

The comedi core calls comedi_buf_reset() before starting an async command
(*do_cmd) and after returning a subdevice to an idle state (*cancel).

The drivers do not need to reset the async->cur_chan in those functions.

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