staging: comedi: me4000: fix me4000_ai_cancel()

The STOP and IMMEDIATE_STOP bits in the ME4000_AI_CTRL_REG should be set,
not cleared, to stop any running conversions.

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>