staging: comedi: me4000: remove unnecessary me4000_ai_cancel()

The comedi core ensures that the subdevice is not busy before it allows
starting a new command. The subdevice (*cancel) is called when the
subdevice is set to not busy. In this driver the me4000_ai_cancel()
is the ai (*cancel) so the extra call in the ai (*do_cmd) is not
necessary. 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>