staging: comedi: das16: remove unnecessary das16_ai_disable()

The das16_ai_disable() function disables interrupts and the pacer clock. This
function is called by the subdevice (*cancel) operation.

The comedi core will not call a subdevice (*insn_read) operation while an
async command is running due to the s->busy check in parse_insn(). Since all
async commands are terminated by a (*cancel), the das16_ai_disable() is not
necessary in das16_ai_insn_read(). Remove it.

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>