staging: comedi: amplc_dio200: support multiple read/write of counter

Change the `insn_read` and `insn_write` handlers of the '8254' counter
subdevices to support reading and writing multiple data values.
(Writing multiple values isn't very useful, but if that's what the user
wants....)

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>