staging: comedi: amplc_pci230: remove PCI230_ADC_CONV

The macro `PCI230_ADC_CONV` is never used.  Just remove it.  (At one
time, the value was written to the `PCI230_ADCSWTRIG` register to
trigger a conversion, but any value would do, and the driver no longer
uses that register to trigger conversions anyway.)

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>