staging: comedi: me4000: clear the ME4000_AI_CTRL_REG in me4000_reset()

Reset the analog input control register after ensuring that any active
conversions have been stopped. This mimics what the ai subdevice (*cancel)
does.

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>